KNOXVILLE, Tenn. -

Jury selection is set to begin in the trial of three peace activists who broke into an Oak Ridge weapons plant last summer.

Megan Rice, Michael Walli and Greg Boertje-Obed face charges that include sabotage. The trio has acknowledged cutting through chain-link fences, spray-painting messages inside the plant and hurling human blood against a wall of the nation's storage center for bomb-grade uranium.

They argue that their actions were necessary because nuclear weapons "threaten the well-being of our entire planet."

The Knoxville News Sentinel reports that in pre-trial motions, U.S. District Judge Amul Thapar ruled the defendants cannot use the necessity defense. Jury selection is Monday and the trial should begin Tuesday.

The felony count under the Sabotage Act carries a maximum sentence of 20 years in prison.