The U.S. government stores weapons-grade uranium at a complex in Oak Ridge, Tenn., considered to be one of the most secure sites in the world.

But last weekend, three aging peace activists—two men and an 82-year-old nun—cut through fences and eluded guards to throw human blood on the building where enriched uranium is stored. The protesters also hoisted banners, recited prayers and read from the Bible.

The security breach prompted the facility, the Y-12 National Security Complex, to stop all nuclear activity Wednesday and go on "security stand down" for the first time since the Sept. 11, 2001, terrorist attacks, while investigators try to figure out how the protesters got past supposedly impenetrable defenses. The temporary stoppage was expected to end by next week.

The nuclear material was never at risk, Steven Wyatt, a spokesman for the complex, said Wednesday.

Lethal force is authorized on the grounds and a series of barbed-wire fences and motion sensors ring the property. "They are very lucky they didn't get shot," Mr. Wyatt said.