The Energy Department said Monday that it was replacing guards and supervisors on duty 10 days ago when three peace activists, including an 82-year-old nun, breached perimeter fences at the principal U.S. facility for storing weapons-grade enriched uranium, according to an article from Reuters.

A federal official at the U.S. Energy Department’s Y-12 complex in Oak Ridge, Tennessee, had also been “temporarily re-assigned” pending the investigation, a government official said in the Reuters article.

Enriched uranium is both processed and stored at the facility, which a senior official had previously declared “the Fort Knox of uranium.” However, Peter Stockton, a former Congressional investigator and security consultant for the Energy Department, said that for years there have been questions about the building’s security, including whether guards’ sight-lines are adequate, the article reports.

The peace activists, 82-year old Sister Megan Rice, 63-year old Michael Walli and 57-year old Greg Boertje-Obed, cut through a number of fences to reach the outer walls of the “Highly Enriched Uranium Materials Storage Facility,” the government’s main stockpile site for bomb-grade uranium, the article says.

The building was designed and built after the September 11, 2001 Al Qaeda attacks with special features to withstand possible militant attacks, but Rice says that while the activists were confronted by as many as 12 guards, “they dribbled in and out … It was very gradual.” She also added: “First the one, and he began to alert others,” Reuters reports.

The activists hung banners and strung crime scene tape on the building, and daubed slogans on the outer walls, the article says. They were arrested and face charges of “willfully and maliciously destroying or attempting to destroy government property.”

As a consequence of the breach, Joshua McConaha, a spokesman for the National Nuclear Security Administration, said that the agency “has taken steps to remove the leadership team and the guard forces on duty at the time, and to replace them with some of the best security experts from around our enterprise.”