Technicians at the government’s Los Alamos National Laboratory settled on what seemed like a surefire way to win praise from their bosses in August 2011: In a high-tech testing and manufacturing building pivotal to sustaining America’s nuclear arsenal, they gathered eight rods painstakingly crafted out of plutonium, and positioned them side-by-side on a table to photograph how nice they looked.

At many jobs, this would be innocent bragging. But plutonium is the unstable, radioactive, man-made fuel of a nuclear explosion, and it isn’t amenable to showboating. When too much is put in one place, it becomes “critical” and begins to fission uncontrollably, spontaneously sparking a nuclear chain reaction, which releases energy and generates a deadly burst of radiation.