An 83-year-old American nun is facing a long prison sentence for breaking into a uranium storage complex to protest against nuclear weapons.

Sister Megan Rice has been convicted along with Michael Walli, 64, and Greg Boertje-Obed, 56, for staging a protest at the facility in Tennessee in July last year.

The Oakridge nuclear weapons plant is known as the 'Fort Knox' of uranium. But that didn't stop Sister Megan from cutting through a security fence and spending two hours inside the complex with her fellow protesters.

Once inside, they hung banners, painted slogans and chipped a wall with hammers. They also sprayed the exterior of the complex with baby bottles containing human blood, the BBC reports.

Defence lawyers said the government was over-reacting to the break-in, because it caused embarrassment.

But prosecutors said it harmed the facility's international reputation - even though there was no chance the protesters would reach the uranium stored inside.

A jury deliberated for two-and-a-half hours before reaching its verdict. The three face up to 20 years in prison after their conviction for sabotaging the plant, which was first constructed during the Manhattan Project that developed the first nuclear bomb.

The three, who belong to the group Transform Now Plowshares, were also found guilty of causing more than $US1000 of damage to government property, for which they could face up to 10 years in jail.

Sister Megan said she only regretted having waited 70 years to take action.