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.
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 in Oak Ridge. The facility holds the nation's primary supply of bomb-grade uranium and was known as the "Fort Knox of 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.
"Please have no leniency with me," she said. "To remain in prison for the rest of my life would be the greatest gift you could give me."
Rice, Greg Boertje-Obed (bohr-CHEE' OH'-bed) and Michael Walli all said God was using them to raise awareness about nuclear weapons and they viewed their break-in as a miracle.
Their attorneys asked the judge to sentence them to time they had already served, about nine months, because of their record of goodwill throughout their lives.
U.S. District Judge Amul Thapar said he was concerned they showed no remorse and he wanted the punishment to be a deterrent for other activists.
On July 28, 2012, the three activists cut through three fences before reaching a $548 million storage bunker. They hung banners, strung crime-scene tape and hammered off a small chunk of the fortress-like Highly Enriched Uranium Materials Facility, or HEUMF, inside the most secure part of complex.
They painted messages such as, "The fruit of justice is peace," and splashed baby bottles of human blood on the bunker wall.
"The reason for the baby bottles was to represent that the blood of children is spilled by these weapons," Boertje-Obed, 58, a house painter from Duluth, Minn., said at trial.
Although the protesters set off alarms, they were able to spend more than two hours inside the restricted area before they were caught.
When security finally arrived, guards found the three activists singing and offering to break bread with them. The protesters reportedly also offered to share a Bible, candles and white roses with the guards.
The Department of Energy's inspector general wrote a scathing report on the security failures that allowed the activists to reach the bunker, and the security contractor was later fired.
Some government officials praised the activists for exposing the facility's weaknesses. But prosecutors declined to show leniency, instead pursing serious felony charges.
At trial and during the sentencing phase, prosecutors argued the intrusion was a serious security breach that continued to disrupt operations at the Y-12 complex even months later.
Attorneys for Rice and Walli, 65, both of Washington, D.C., said the protesters were engaged in a symbolic act meant to bring attention to America's stockpile of nuclear weapons, which they view as both immoral and illegal under international law.
"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," Boertje-Obed's wife, Michele Naar-Obed, said before the hearing.
Rice testified at trial that she was surprised the group made it all the way to the interior of the secured zone without being challenged and that plant operations were suspended.
"That stunned me," she said. "I can't believe they shut down the whole place."
They were found guilty on May 8, 2013, of sabotaging the plant and damaging federal property.
At the first part of the sentencing hearing three weeks ago, more than 100 supporters filled the courtroom and an overflow room where they watched the proceedings on a video feed. Friends of the defendants testified to their good characters and kind hearts, saying the three had dedicated their lives to pursuing peace and serving the poor.
That hearing was abruptly shut down when the federal court house was closed because of snow, but not before U.S. District Judge Amul Thapar ruled the three had to pay combined restitution of nearly $53,000 for their actions.
About 75 supporters attended the sentencing hearing Tuesday.