WASHINGTON (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.

While moving between the perimeter fences, the activists triggered sensors that alerted security personnel. But officials conceded the intruders were still able to reach the building’s walls before security personnel got to them.

Ellen Barfield, a spokeswoman for the activists who called themselves “Transform Now Plowshares,” said three were arrested and charged with vandalism and criminal trespass.

She said the three, identified as Megan Rice, 82, Michael Walli, 63 and Greg Boertje-Obed, 57, were being held in custody and appeared for a hearing before a U.S. magistrate judge in Knoxville, Tennessee, on Thursday.

A detention hearing is set for Friday afternoon, when prosecutors must show the defendants are a flight risk and a danger to the community in order to keep them in custody, according to court officials. The trial date is October 9.

Barfield forwarded a statement from the group in which it said the activists had passed through four fences and walked for “over two hours” before reaching the uranium storage building, on which they hung banners and strung crime-scene tape.

Ralph Hutchinson, coordinator for the Oak Ridge Environmental Peace Alliance, said the group’s intention was not to demonstrate the lack of security at the plant, but to take a stance against the making of nuclear weapons.

“It wasn’t so they could show how easy it was to bust into this bomb plant, it was because the production of nuclear weapons violates everything that is moral and good,” Hutchinson said. “It is a war crime.”

NUCLEAR MATERIALS ‘NOT COMPROMISED’

Officials said that the storage building itself, which was built after the September 11, 2001, al Qaeda attacks on New York and Washington, was designed with modern security features and that its contents were not compromised.

WSI Oak Ridge, the private firm employed by the U.S. Department of Energy to provide security at Y-12, is a subsidiary of the giant international security firm G4S.

G4S drew criticism for failing to provide the number of security personnel it promised to protect the London Olympic Games, forcing the British government to deploy extra army troops.

A spokeswoman for G4S declined to comment and referred inquiries to government spokespeople.

The security failure was an embarrassment both for the security firm and for the National Nuclear Security Administration, or NNSA, the Energy Department branch that operates U.S. nuclear weapons plants. “It was obviously a pretty serious incident,” NNSA spokesman Joshua McConaha told Reuters.

“We’re taking this very, very seriously,” added Steve Wyatt, a spokesman for the NNSA office in Oak Ridge, which supervises the activities of Y-12 contractors.

The NNSA officials said the activists cut through two chain-link fences surrounding the sprawling facility and a third fence surrounding the ultra-secure enriched uranium stockpile building, known as the “Highly Enriched Uranium Materials Facility.”

Wyatt said the building served as the U.S. government’s only “warehouse” for storing highly enriched uranium used in nuclear weapons.

Highly enriched uranium is a radioactive material used in the core of bombs to produce a nuclear detonation. The Oak Ridge plant is one of the most important government installations involved in the maintenance and production of the U.S. nuclear arsenal.

INCIDENT REVEALED NUCLEAR RISKS

Although the security breach occurred overnight last Friday, officials confirmed that the shutdown - which applies to “all nuclear operations” at the Y-12 site - did not begin until Wednesday. Officials said it was expected to continue into next week.

In the meantime, personnel at the facility would be given additional security training.

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.