(Reuters) - The U.S. government's only facility for handling, processing and storing weapons-grade uranium has been temporarily shut after anti-nuclear activists, including an 82-year-old nun, breached security fences, government officials said on Thursday.

WSI Oak Ridge, the contractor responsible for protecting the facility at Oak Ridge, Tennessee, is owned by the international security firm G4S, which was at the center of a dispute over security at the London Olympic Games.

Officials said the facility was shut down on Wednesday at least until next week after three activists cut through perimeter fences to reach the outer wall of a building where highly enriched uranium, a key nuclear bomb component, is stored.

The activists painted slogans and threw what they said was human blood on the wall of the facility, one of numerous buildings in the facility known by the code name Y-12 that it was given during World War Two, officials said.

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"Peter Stockton, a former congressional investigator and security consultant to the Energy Department, expressed skepticism at government assertions the nuclear material was not at risk.

"It is unbelievable this could happen," Stockton said. "The significance is outrageous. If they were terrorists, they could have blown open the door and got inside."

Stockton said the security breach was the "worst we've ever seen." He said it was more serious than the case of Wen Ho Lee, a Taiwan-born scientist who was suspected of espionage at the Los Alamos nuclear laboratory. He pleaded guilty in 2000 to a less severe charge when the case against him collapsed."

TLS- So imagine that the same lame security company G4S that got caught in the UK providing crap security for the Olympics is the same outfit protecting our warehouse of fissile nuclear material... Tell me again why the U.S. military cannot do this job and we're hiring private firms to do this? Had they been lets say REAL terrorists with a few duffel bags of C-4 and a few rifles to fight off guards. They could have blasted their way in then grabbed a bunch of HEU, affixed C-4 blocks or HMX around it carelessly. Then with a synchronous all at once detonator connected to each block detonated it. Very likely it would have gone nuclear even done hastily. It would have vaporized ALL the nuclear material in that warehouse and likely surrounding warehouses producing one of the most radioactive mushroom clouds imaginable. We're talking evacuate the east coast shit... WOW so they just walked up to THAT building?  I can't imagine what other locations we have dumb ass security company's protecting?