In 1946, shortly after the end of World War II, the physicist Louis Slotin stood in front of a low table at the Los Alamos National Laboratory, concentrating intensely on the object in front of him. His left thumb was hooked into a hole on the top of a heavy beryllium dome, fingers bracing the side as he carefully cantilevered it on its leftmost edge. In his right hand he held a flathead screwdriver, its head wedged under the right edge of the dome to keep it from closing completely. Through the gap on the right side you could just barely catch a metallic gleam, a glimpse of the 14-pound plutonium sphere that was slated to become one of the United States’ next nuclear weapons.

Slotin began slowly lowering the dome, using the screwdriver gingerly to control the opening. He had performed this feat many times before, but this time was different. As his left hand eased its hold on the dome, his right hand slipped outward just a hair too far, pulling the screwdriver out from under its edge and leaving nothing to stop it from closing.

A bright blue flash signaled that the dome had fallen into place, and Slotin felt an intense heat all across his skin. He quickly twisted his wrist to pop the dome completely off the plutonium sphere, but the damage had already been done. Nine days later, he was dead.

In the half-second that the dome was closed, the plutonium had gone supercritical, initiating a nuclear chain reaction and releasing a lethal dose of gamma rays that damaged Slotin’s cells beyond repair. Seven other people were in the room with him during the accident; three were hospitalized for acute radiation sickness. All it had taken was misplacing a screwdriver a fraction of an inch, for a fraction of a second. Richard Feynman called the technique, relatively common at the time, “tickling the tail of a sleeping dragon.”

The demon core, that 14-pound lump of plutonium that claimed the life of Louis Slotin, began its existence as rods of uranium-238, a relatively stable isotope, at the Hanford Site in Washington State. These rods were inserted into a nuclear reactor and bombarded with neutrons, tiny, uncharged subatomic particles, with the hope that some would get stuck to uranium atoms, increasing their atomic number to 239. Unlike uranium-238 (a half-life of 4.5 billion years), uranium-239 is very unstable (23 minutes); it rapidly decays into neptunium-239 (2.4 days), and after that, plutonium-239 (24,000 years). The crude plutonium for the demon core was then purified and sent to the Los Alamos Laboratory as a salt, plutonium nitrate.

Going from plutonium nitrate to a finished bomb core had proven to be a major challenge for the metallurgists at Los Alamos Laboratory. By the time they made the demon core (the third plutonium core, after those in the Trinity and Nagasaki bombs) they had worked out many of the kinks. They first converted the plutonium salt into plutonium metal by reacting it with metallic calcium, a relatively straightforward process. The metallurgists weren’t done, though: Early studies had found that plutonium produced this way is incredibly brittle, cracking with every attempt to make it into the needed shapes. To solve this, the metallurgists tried mixing the plutonium with various other metals to see if they could improve its workability as an alloy, eventually finding gallium to be ideal. The plutonium-gallium alloy of the demon core was hot-pressed into two hemispheres and then coated with a thin layer of nickel to protect the plutonium from rusting; joining these two hemispheres together completed its manufacture.

A recreation of the plutonium-handling accident that killed Louis Slotin (Los Alamos National Laboratory)

When and how World War II would end was still uncertain, so the scientists at Los Alamos had worked tirelessly to complete the core. Just as they were preparing to ship it for final installation in a bomb, the August 15, 1945, surrender of Japan came. This development left the U.S. Nuclear Program with a question: What would be done with the heart of the third bomb? Mercifully, it was no longer needed for warfare.

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.

In the early days of nuclear-fission bombs, that danger was palpable and understood. Not just because those bombs could level cities and murder populations, but because their components were still new and unproven, offering reason to respect and fear them. Today, nuclear warheads go unseen and unconsidered, even as nuclear war feels closer than it has in decades.

Los Alamos ended hand-manipulation of nuclear cores in criticality experiments after Slotin’s death. That was certainly for the best. And yet, Feynman’s quip about the test—comparing it to tickling the tail of a sleeping dragon—also kept the certainty of unimaginable destruction close at hand.