The resulting blue glow — known as Cherenkov radiation — has accidentally and abruptly flashed at least 60 times since the dawn of the nuclear age, signaling an instantaneous nuclear charge and causing a total of 21 agonizing deaths. So keeping bits of plutonium far apart is one of the bedrock rules that those working on the nuclear arsenal are supposed to follow to prevent workplace accidents. It’s Physics 101 for nuclear scientists, but has sometimes been ignored at Los Alamos.

As luck had it that August day, a supervisor returned from her lunch break, noticed the dangerous configuration, and ordered a technician to move the rods apart. But in so doing, she violated safety rules calling for a swift evacuation of all personnel in “criticality” events, because bodies — and even hands — can reflect and slow the neutrons emitted by plutonium, increasing the likelihood of a nuclear chain reaction. A more senior lab official instead improperly decided that others in the room should keep working, according to a witness and an Energy Department report describing the incident.

Catastrophe was avoided and no announcement was made at the time about the near-miss — but officials internally described what happened as the most dangerous nuclear-related incident at that facility in years. It then set in motion a calamity of a different sort: Virtually all of the Los Alamos engineers tasked with keeping workers safe from criticality incidents decided to quit, having become frustrated by the sloppy work demonstrated by the 2011 event and what they considered the lab management’s callousness about nuclear risks and its desire to put its own profits above safety.