Nun, 82, exposes the weakness at the heart of 'Atomic City'

Out of America: Security breach turns spotlight on secret plant in Tennessee where US stores uranium for bombs

Sister Act: Sister Megan Rice, Michael Walli and Gregory Boertje-Obed are interviewed by local media after appearing in federal court in Knoxville (Getty Images)

Since security is on everyone's mind here after the Boston marathon bombs, consider this question: what hope is there of keeping America safe, if an 82-year-old Catholic nun and a couple of mild-mannered peace activists can break into the country's most important nuclear weapons facility, armed with nothing more formidable than a bolt cutter?

A partial answer may be provided by the trial of Sister Megan Rice, house painter Greg Boertje-Obed, and Michael Walli, described as a "drifter", which opens on Tuesday in Knoxville, Tennessee. They face charges of damaging government property at the Y-12 enriched-uranium complex at Oak Ridge, a few miles from Knoxville, and – considerably more serious – of "intent to injure, interfere with and obstruct national defence", in other words, sabotage.

The whole thing, under the federal criminal code, adds up to a massive potential fine and a possible 20 years or more in jail. But that, of course, is not the view of Sister Megan and her colleagues, perpetrators of what must be one of the worst nuclear security breaches in US history. In their eyes, they were on a divine mission to rid the world of such weapons, and therefore not the real defendants in the case. On trial instead should be Oak Ridge, and everything it stands for.

Think of the Manhattan Project, and the bombs that obliterated Hiroshima and Nagasaki, and you think of the Los Alamos research laboratories. You think of Robert Oppenheimer and the brilliant physicists who worked there, of the Trinity test in July 1945, and of Oppenheimer's famous quotation (delivered many years later, it should be said) from the Hindu scriptures: "Now I am become Death, the destroyer of worlds."

But if the boffins were in the remote mountains of New Mexico, the material they needed was being produced in a green valley of north-eastern Tennessee. The remarkable story of Oak Ridge has been told in a new book by Denise Kiernan: The Girls of Atomic City: the Untold Story of the Women who Helped Win World War II. Kiernan focuses on the perspective of the tens of thousands of female workers brought in 70 years ago to work at this new and top-secret project, but its nickname is as true now as then. From the time it sprouted like a colossal mushroom of brick and steel from the Tennessee soil, Oak Ridge has been a synonym for matters atomic.

The first eviction notices to families and farmers arrived from the US War Department in late 1942, giving them three weeks to move. Within a couple of years, their land had become a town of 75,000 people, operating round the clock and using more electricity than New York City. However, it was marked on no map.

Ringed by 95 miles of fence and patrolled by 1,000 guards, Oak Ridge was a closed city. It might have been the fifth largest settlement in Tennessee but officially it did not exist. With so many men away on war duty, women formed a large part of the workforce, doing everything from clerical work to running the calutrons that separated the fissile U-235 isotope. If they discussed their work with a colleague, they risked instant dismissal. Few had the slightest notion of what was going on. Even those with suspicions spoke merely of "the gadget".

But on 6 August 1945, when the sun rose twice over Hiroshima, the secret was no more. "Atomic superbomb, made at Oak Ridge, strikes Japan", blared the headline of that evening's edition of The Knoxville News-Sentinel. By 1949, Oak Ridge was to be found in atlases, a normally incorporated city. But the local business has not changed.

Today's nuclear complex, run by the Department of Energy, covers more than 50 square miles. And Y-12 is still there, rebuilt and probably the biggest single repository of nuclear materials and know-how on earth. It serves as the US government's main storehouse of highly enriched uranium – enough, it is said, to build 10,000 nuclear warheads.

You have to wonder whether the Girls of Atomic City would have had regrets, had they known what precisely they were working on. Probably not, given their excitement at being involved in the war effort. But of Sister Megan's feelings, in her decades of campaigning, there has never been the slightest doubt.

By the time she, Walli and Boertje-Obed penetrated the Y-12 complex in the early hours of 28 July 2012, she had already been arrested 50 times and spent six months in federal prison as a result of such protests. "If I was called upon to die for this truth I would certainly be willing to die," she told a recent interviewer, "for such a message to the world that we must stop this killing."

The penetration itself proved ridiculously easy. With their bolt cutter the trio chopped through four chain-metal fences, the last three of them warning of a death-zone. But though alarms eventually went off, nothing happened for 20 minutes until a lone guard appeared, later joined by others. In the meantime, they splashed blood on the walls of Y-12, spray-painted slogans, and recited prayers for peace.

In Washington there was consternation, and vast embarrassment: a frail nun and two men in late middle age are one thing; three fanatical terrorists quite another. America has been transfixed by a couple of Chechen kids who used ordinary pressure cookers to cause mayhem in Boston: what if the Tsarnaev brothers had done some serious "dirty bomb" homework on the internet, and gone after Y-12?

"The nuclear bomb," said Sister Megan when she was released from custody five days after the break-in, "is the worst weapon in the history of mankind. It should not exist." But it does. And nowhere more so than at Oak Ridge.