A nun (84) who broke into a US nuclear plant and defaced a bunker holding bomb-grade uranium has been jailed for three years.

Sister Megan Rice, along with two other men, cut through three fences and hung banners, painted slogans and splattered human blood on the building.

The incident on July 28, 2012, was highly embarrassing for officials running the Y-12 National Security Complex in Oak Ridge, Tennessee.

The facility plays a role in making, maintaining and dismantling parts of every weapon in the US nuclear arsenal.

No danger

However, officials said there was never any danger of the group getting their hands on nuclear materials.

The other two activists, Greg Boertje-Obed and Michael Walli, received sentences of more than five years.

The trio spent two hours inside the complex before being caught.

A jury decided last May that they were all guilty of a charge of interfering with national security and a second charge of damaging federal property.

They had faced potential sentences of up to 30 years.

No regrets

"The shortcomings in security at one of the most dangerous places on the planet have embarrassed a lot of people," lawyer Francis Lloyd told the trial.

Sister Rice said in her closing statement her only regret was that she waited so long to do it.

"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," she added.