An opportunity came in the form of Operation Crossroads, a testing program to determine the impact of nuclear weapons on warships. However, now that the wartime pressure was off, the scientists realized that it would be useful to run some more tests on their creation.

The demon core, destined for use in a weapon of mass destruction, was designed to have a hair trigger. It was meant to be in a “-5 cents” configuration, meaning that it would take only 5 percent more plutonium for the core to go supercritical and result in a radiation accident. In other words, the core was always right on the edge of going off. A few errant neutrons would be enough to trigger a deadly chain reaction. The scientists knew that the core was unstable, though now they had the opportunity to measure just how close it was to going critical and how, with different techniques, it might be brought even closer.

The experiment that killed Louis Slotin in 1946 posed these questions. It was already well-known that bombarding a plutonium core with neutrons could provide the push it needs to go critical, and it was also known that plutonium fires off neutrons as part of its natural decay. By partially covering the core with a dome made out of beryllium, an excellent neutron reflector, it was possible to bombard the core with enough of its own neutrons to bring it close to criticality, while still allowing enough to escape to keep it from going all the way. By dropping the dome, Slotin had stopped any neutrons from escaping, giving the demon core enough energy to undergo a dangerous chain reaction.

Slotin was actually the second scientist to be killed by the demon core. Harry Daghlian, another physicist at Los Alamos, had attempted a similar experiment the previous year using tungsten-carbide bricks instead of a beryllium dome. While arranging the bricks around the core, he accidentally dropped one right on top of it, supplying that small amount of energy needed to go critical. Like Slotin, Daghlian acted quickly to remove all the neutron reflectors from the demon core, but it only takes a moment of supercriticality to release a lethal amount of radiation. Daghlian wasted away in the hospital for 25 days before finally succumbing to his injuries.

The demon core never actually made it into a nuclear weapon. Following the Slotin accident, it was melted down and its elements were redistributed across several new warheads. Still, its existence offered an important lesson: Even outside a weapon, plutonium posed an immediate danger. All it took was an errant brick or misplaced screwdriver to kill two civilian scientists.

Today, an estimated 15,000 nuclear weapons are spread across the globe. That figure should also suggest a much larger danger. All it would take is a misread message or mis-pressed button to kill millions of people, potentially igniting a global conflict that would kill millions more.