An 84-year-old nun was sentenced to three years in prison on Tuesday for breaking into an American nuclear weapons complex and defacing a building that stored weapons-grade uranium.

Megan Rice, along with two other activists, Greg Boertje-Obed and Michael Walli, each received five-year prison terms after successfully penetrating the security of the Y-12 National Security Complex in Oak Ridge, Tennessee on 28 July 2012.

Despite setting off alarms, they successfully evaded security for two hours.

The complex had previously been described as the "Fort Knox" of uranium.

The activists cut through three fences to reach a storage bunker that had cost $548 million to construct.

They hung protest banners, strung crime-scene tape, and hammered off a small chunk of the fortress-like Highly Enriched Uranium Materials Facility.

Messages such as "the fruit of justice is peace" and baby bottles of human blood were splashed onto the bunker's wall.

Quoted by AP, Mr Boertje-Obed, a 58-year-old house painter from Duluth, Minnesota explained: "The reason for the baby bottles was to represent that the blood of children is spilled by these weapons."

The guards that eventually caught Ms Rice found her and her companions singing.

They welcomed the security officers and offered to share communion with them, as well as giving them gifts of white roses, candles and a Bible.

The complex had to be shut down as a result of the break in.  Security personnel were given new training and new contractors were brought in.

Ms Rice and Mr Walli's lawyers explained that they were attempting a symbolic protest to draw attention to the US's nuclear weapon stockpile, which they viewed as both immoral and a violation of international law.

Mr Walli's lawyer described to the court how he had served two tours in Vietnam before returning to the US and dedicating his life to peace and charity. Mr Walli said he had no remorse about the break-in and would do it again.

"I was acting upon my God-given obligations as a follower of Jesus Christ," he told US District Judge Amul Thapar.

Speaking to AP, Mr Boertje-Obed's wife, Michele Naar-Obed said that while she would manage with whatever sentence he received, she only hoped that his efforts had not been in vain.

"What I'm hopeful for is that people really could appreciate what he did and why he did it and who he did it for. He did it for all of us," Ms Naar-Obed said.

Ms Rice is a sister in the Society of the Holy Child Jesus. After becoming a nun at the age of 18, she served for 40 years as a missionary in western Africa teaching science.

In her closing statement, she requested the judge sentence her to life in jail, although the sentencing guidelines suggested the appropriate sentence was six years.

"Please have no leniency with me," she was quoted by AP as saying. "To remain in prison for the rest of my life would be the greatest gift you could give me."

Speaking about her cause, she said: "This is the next generation and it is for these people that we're willing to give our lives."

The judge was concerned that the demonstrators showed no remorse for their actions, and wanted their punishment to be a deterrent for other activists.

The higher sentences for Mr Walli and Mr Boertje-Obed were linked to their much longer histories of mostly nonviolent civil disobedience.

The judge was also notably skeptical about the existence of any real harm done by the elderly nun and her friends and challenged prosecutors to prove it.

US Attorney Jeff Theodore who was quoted by AP as saying that the most substantial damage was the destruction of the "mystique" of the facility.

While some officials praised the activists for exposing the facility's weaknesses, prosecutors declined to show leniency, instead pursuing serious felony charges.

During the proceedings, the courtroom was filled with 75 supporters and an overflow room with a video linkup was needed.

On top of prison sentences, Ms Rice and her co-activists are required to spend an additional three years of supervised release, and together they must pay over $50,000 in restitution.