Story highlights Sister Megan Rice, 84, was found guilty of damaging and destroying government property

She and two other peace activists were arrested in 2012 for breaking into nuclear facility

The uranium processing and storage complex was supposed to have been tightly secured

An 84-year-old nun was sentenced to 35 months in prison Tuesday for breaking into a nuclear facility, her lawyer said.

In May, a federal jury in Knoxville, Tennessee, found Sister Megan Rice; Greg Boertje-Obed, 57; and Michael Walli, 63, guilty of destroying U.S. government property and causing more than $1,000 in damage to federal property.

It was not immediately clear what the sentences were for the two other peace activists.

The incident began before dawn on July 28, 2012, when the three cut through a chain-link fence surrounding the Y-12 National Security Complex in Oak Ridge, Tennessee.

They then walked nearly a mile, cutting through three more fences and breaching what was supposed to be the most tightly secured uranium processing and storage facility in the country.

It was not until hours later that a guard finally confronted the activists who, by then, had hoisted banners, spray-painted messages and splattered human blood on a building that houses highly enriched uranium.

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"They're at peace about this. They're peacemakers, and they knew that they risked this," Joe Quigley, attorney for Walli, told CNN affiliate WATE in May after their trial. "Nobody is happy to go jail, but they understand."

In response to the incident, Congress has held a series of hearings and issued security recommendations to the Department of Energy and its National Nuclear Security Administration, which runs Y-12 and seven other nuclear weapons sites.

In March, Deputy Energy Secretary Daniel Poneman testified before the House Energy and Commerce Committee that the Department of Energy had taken "several major actions ... to improve security" since the breach, including management changes and independent security reviews.