In 1946, a P-239 plutonium core scheduled for detonation was harmlessly melted down and reintegrated into the United States’ nuclear stockpile. That was the end of a 14-pound metallic sphere that had killed two scientists not 11 months before – the Demon Core.

You could have picked up the spheres of atomic material destined for the bombs that obliterated Hiroshima and Nagasaki. Both “Little Boy” and “Fat Man” used sub-critical masses of uranium and plutonium, respectively. The cores would be warm to the touch, but the nickel-plating that covered WWII’s nuclear cores would prevent much of the escaping radiation from harming you. The terrible explosions that these bombs are famous for only happen when the cores go super-critical – using explosives to artificially induce a “critical mass” situation where neutrons escaping one atom would knock other atoms around it and cause them to do the same. At the heart of an atomic blast is a core filled with atoms poised as rat-traps and mechanisms that force one to snap.

The Demon Core was the third core of fissionable nuclear material made during the WWII era. It was also to be dropped on Japan, but after the empire’s prompt surrender following the bombing of Nagasaki, the core had no immediate military use. Instead, a physicist working at the Los Almos National Laboratory in New Mexico after the end of war decided to use the remaining core to figure out just what it takes to make a core go critical.

When Otto Robert Frisch first arrived in the United States, he was placed at Los Almos to study neutron multiplication in uranium and plutonium. In other words, these elements naturally radiated neutrons in the process of nuclear decay, but if there is enough of the radioactive element close enough together, these neutrons can cause the rat-trap-style chain reaction as neutrons jiggle more neutrons free from other atoms. However, there is another way to force criticality other than just having a bunch of plutonium. You can reflect neutrons coming out of the material back in on itself like a mirror with a thicker material until a self-sustaining reaction occurs. Even though an explosion might not happen after criticality is reached, a deadly blast of radiation certainly could.

And that’s exactly what happened.

By 1944, Frisch was leading the Critical Assembly Group at Los Almos and started a dangerous series of near-criticality tests. Commenting on the risk, physicist Richard Feynman reportedly said that the experiments were “like tickling the tail of a sleeping dragon.” With the ever-present possibility of being roasted by radioactive dragon fire, the “Tickling the Dragon’s Tail” experiments continued. Within two years, the neutron reflection experiments on the Demon Core would claim the lives of two scientists, Harry K. Daghlian, Jr. and Louis Slotin.

A recreation of the criticality test that Daghlian conducted before his death.

On August 21^st, 1945, 24-year old Harry K. Daghlian, Jr. was conducting a criticality experiment with the Demon Core. He was placing tungsten-carbide bricks around the mass of plutonium to see how many bricks it would take – and in what orientation – to reflect enough neutrons and cause the core to go critical. That afternoon, using a Geiger counter-like device to measure the radiation coming from the plutonium, he got close enough to criticality that he decided to end the experiment. The result should have been just a few graphite streaks in a notebook. But Daghlian was curious. Later that night he returned, alone, to his assembly for another test.

Again, Daghlian created an orientation of bricks that his measuring device told him was nearly critical. As he was placing one last brick, the measuring device indicated that the core would go super-critical if he completed the assembly – the dragon would wake up. Cautiously, he began removing the final brick. Then he dropped it.

The moment the brick hit the assembly, the Demon Core went super-critical. According to Daghlian, there was a blast of blue light and a wave of heat. Daghlian reacted quickly and used his right hand to knock the dropped brick to the floor. In those few moments, he received a fatal dose of radiation. The core calmed down, but 25 days later Daghlian fell into a coma and died from severe radiation poisoning.

A security guard, Private Robert J. Hemmerly, sitting at his desk twelve feet away reading his newspaper died 33 years later from what was considered radiation-caused leukemia.

Harry K. Daghlian, Jr. was the first official fatality from a criticality accident. He arranged to have his body donated to science after his inevitable end, so that scientists could study the effects of radiation exposure on the human body. During the 25 days after his fatal mistake, Daghlian’s colleague Louis Slotin spent many hours at his bedside, comforting the 24-year old as radiation poisoning claimed him. Exactly seven months later, in the same hospital, Slotin would suffer the same fate.

A recreation of the criticality experiment that Slotin conducted.

By 1946, Louis Slotin had taken over the Critical Assembly Group from Frisch. Though the criticality experiments were moved from the “Omega Site” at Los Almos to the Palajito laboratory, the dangerous nature of the experiments continued. It seemed that legendary physicists had the right idea. In addition to Feynman’s warnings, Enrico Fermi warned Slotin that he would be dead within a year if he continued the criticality experiments.

On May 21^st, 1946, Slotin was conducting another criticality experiment with the Demon Core. This time, instead of tungsten-carbide bricks, Slotin was testing the critical point of the core by placing it inside two half-spheres made of beryllium and lowering them until there was just a sliver of space before the sphere was complete. If the core was completely enclosed, the reflection of neutrons would be total, and the dragon would wake again.

The local expert among experts, Slotin was reportedly brash in his experimental protocols. In a pair of blue jeans and cowboy boots, Slotin would perform the criticality tests without the necessary spacers that would keep the sphere from completely closing. Instead, he would put his bare hand on a thumbhole on the top half-sphere and attempt to lower it using only the blade of a flat-head screwdriver. That way he could get closer to criticality, and had completed the experiment successfully a dozen times.

A diagram of the positions of the scientists at the time of Slotin’s accident.

The afternoon of the accident, Slotin was conducting the same experiment as seven colleagues watched. But for whatever reason, this time the screwdriver slipped. The instant the spheres touched, the Demon Core went “prompt critical,” or the reflected neutrons coming from the plutonium inside the sphere initiated super-criticality. Like the Daghlian accident, there was a flash of blue light and a wave of heat before Slotin could flip the top half-sphere off of the assembly. The seven others in the room ran out as the accident occurred, but Slotin called them all back to establish their positions – by knowing their relative positions he could calculate how much radiation they absorbed, and how much their lives had been shortened.

According to physicist Raemer Schreiber, who was present in the room with Slotin, Slotin’s first words immediately after the incident were “Well, that does it.” He died of severe radiation poisoning nine days later at the age of 35 in the same hospital as Daghlian, on another Tuesday the 21^st, as a result of an accident with the same hunk of radioactive metal. It was supposed to be the final demonstration with the core before detonation. It was.

In those few moments, Louis Slotin had been exposed to possibly over 1,000 rads of radiation from the neutrons and gamma rays blasting out of the Demon Core, possibly the highest dose of radiation anyone has ever taken. For comparison, 1,000 meters from ground zero in Hiroshima, the neutron radiation was measured as 400 rads.

Since 1945, over 60 criticality accidents have been recorded, leading to at least 21 deaths in facilities and labs around the world. The flash of blue light that so often accompanies these incidents is an indication of the invisible radiation released. As high-energy particles crash through the air, they excite the molecules within it. The only way for these molecules to return to a lower energy state is to release some radiation of their own: photons of light.

But while gases can absorb and re-emit the energy imparted to it by nuclear material, our bodies can not. Instead, particles and gamma rays flying out of nuclear cores rip electrons from the molecules that make us up and smash our cellular machinery beyond repair. When enough molecular damage is done, radiation sickness can manifest itself as everything from vomiting and diarrhea to blisters, inflammation, impaired cognition and organ failure.

After Slotin’s death, all hands-on criticality experiments ended at Los Almos.

Crossroads Able, a 23-kiloton air-deployed nuclear weapon detonated on July 1, 1946.

Five weeks after Slotin’s accident on July 1^st, 1946, another core was destroyed in a 23-kiloton air-deployed nuclear weapon over Bikini Atoll. It was supposed to be the end of the Demon Core. The core was instead melted down and redistributed, getting a new half-life.

Images: Los Alamos National Laboratory; Navy photographer

Kyle Hill is the Science Editor of Nerdist Industries. Follow on Twitter @Sci_Phile.

*CORRECTION: An earlier version of this article claimed that the Demon Core was detonated in Operation Crossroads, a common misconception surrounding the core’s fate.