AN 84-year-old nun has been sentenced to nearly three years in prison for breaking into a US nuclear weapons complex and defacing a bunker holding bomb-grade uranium, a demonstration that exposed serious security flaws.

Two other activists who broke into the facility with Megan Rice were sentenced to more than five years in prison, in part because they had much longer criminal histories of mostly non-violent civil disobedience.

Although officials claimed there was never any danger of the protesters reaching materials that could be detonated or made into a dirty bomb, the break-in raised questions about the safekeeping at the Y-12 National Security Complex.

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The Tennessee site holds the nation's primary supply of bomb-grade uranium.

After the protest, the complex had to be shut down, security forces were re-trained and contractors were replaced.

In her closing statement, Rice asked the judge to sentence her to life in prison, even though sentencing guidelines called for about six years.

She said: "Please have no leniency with me. To remain in prison for the rest of my life would be the greatest gift you could give me."

Rice, Greg Boertje-Obed and Michael Walli all said God was using them to raise awareness about nuclear weapons and they viewed their break-in as a miracle.

They had been found guilty of sabotaging the plant and damaging federal property in July 2012.