Why Tickling the Dragon's Tail Was the Deadliest Gamble in Science

It sounds like a joke. "Tickling the dragon's tail." Honestly, the name is so whimsical it almost hides the fact that it was a shorthand for staring into the face of a nuclear apocalypse from a few inches away. This wasn't some abstract theoretical exercise performed behind thick lead glass. We’re talking about physicists in the 1940s—men like Louis Slotin and Harry Daghlian—who were literally holding the core of an atomic bomb together with their hands and a couple of screwdrivers.

They were playing with criticality.

In the high-stakes environment of Los Alamos during and immediately after the Manhattan Project, "tickling the dragon's tail" meant bringing a subcritical mass of plutonium or uranium right to the edge of a self-sustaining nuclear chain reaction. If you do it right, you get invaluable data about how a bomb will behave. If you slip? You get a blue flash of light, a wave of heat, and a death sentence that takes days to carry out.

The Demon Core: A Name Earned in Blood

Before it was the "Demon Core," it was just a 6.2-kilogram sphere of plutonium-gallium alloy. It was meant to be the heart of a third atomic bomb dropped on Japan, but the surrender changed those plans. Instead, it stayed at Los Alamos. It became a lab rat.

The goal of these experiments was simple but terrifyingly precise: find out how close you could get to a "critical" state—where the neutrons being produced by the plutonium are perfectly balanced by those escaping—before it goes "prompt critical." When a mass goes prompt critical, the reaction accelerates exponentially in a fraction of a second. It releases a burst of ionizing radiation that shreds human DNA like a hot wire through wax.

Harry Daghlian was the first to feel the dragon's bite. On August 21, 1945, he was working alone—a massive breach of protocol, by the way—building a wall of tungsten carbide bricks around the core to reflect neutrons back into it. He dropped a brick. It landed right on the sphere. He panicked and tried to knock the brick off by hand, stopping the reaction but absorbing a lethal dose of radiation in the process. He died 25 days later. You'd think that would have been the end of it. It wasn't.

Louis Slotin and the Screwdriver Incident

If Daghlian’s death was a tragic accident, Louis Slotin’s was a masterclass in "cowboy science." Slotin was brilliant, sure. He was also arrogant. He had become the lead "tickler" at Los Alamos, often performing the experiment in blue jeans and cowboy boots while chewing on a piece of gum.

On May 21, 1946, Slotin was showing a group of seven other scientists his technique. He was using a beryllium tamper—a hemispherical shell—to cover the core. The idea was to lower the shell slowly to reflect neutrons. To keep it from closing completely (which would trigger criticality), he was supposed to use shims.

He didn't use the shims.

Instead, he used the tip of a flathead screwdriver. He held the shell with his thumb and used the screwdriver to manually gap the distance. It was a manual, hand-eye coordination task with a nuclear weapon. The screwdriver slipped.

The shell slammed shut.

The room filled with a blue glow. That glow isn't actually the radiation itself; it’s Cherenkov radiation or the ionization of the air, a visual signal that the environment has just become incompatible with human life. Slotin jerked his hand up, flipping the shell off and stopping the reaction, likely saving the lives of the seven other people in the room. But he knew. He reportedly turned to his colleague Alvin Graves and simply said, "Well, that does it."

What Science Gained from the Terror

Why did they do it? It seems insane now, especially with our modern obsession with "Safety Third" (or first, depending on who’s asking). But back then, they didn't have the sophisticated computer modeling we use today. You couldn't just simulate a Monte Carlo neutron transport on a MacBook.

The only way to know the exact "critical assembly" of a specific batch of plutonium was to physically assemble it. These men were trying to define the margins of safety for the entire nuclear arsenal of the United States. They needed to know exactly how much material was needed to make a bomb go off and, more importantly, how much was "safe" for soldiers to handle in the field.

The data Slotin and Daghlian died for helped refine the designs of the weapons that defined the Cold War. It led to the development of remote-handling facilities. After Slotin’s death, the "hands-on" era of nuclear physics ended abruptly. Los Alamos moved these experiments to the Pajarito Site, where they used machines like "Godiva"—a bare-metal reactor that could be pulsed safely from a distance.

The Biology of a Criticality Accident

When you tickle the dragon’s tail and lose, you don't die instantly. That’s the most horrific part. The radiation dose Slotin took was estimated at 1,000 rads of neutrons and 114 rads of gamma rays. For context, 400-500 rads is usually enough to kill you.

His body didn't vaporize. Instead, the radiation acted like a billion microscopic bullets, shattering his chromosomes. His cells could no longer divide. His skin began to blister from the inside out. His gastrointestinal lining simply disintegrated. It’s what doctors call a "three-dimensional sunburn."

The nine days Slotin spent dying were a gruesome preview of what a full-scale nuclear war would look like for the survivors. He was conscious for much of it, experiencing what is known as the "walking ghost phase"—a brief period where the victim feels slightly better before the final, total organ failure sets in.

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Why We Still Talk About It

The Demon Core was eventually melted down and recycled into other cores. It’s a bit chilling to think that parts of that specific sphere might still be sitting in a silo somewhere in North Dakota or inside a submarine in the Pacific.

But the legacy of tickling the dragon's tail isn't just about the core itself. It’s about the shift in how we handle high-risk technology. It was the birth of "safety culture" in the nuclear age. It taught us that no matter how smart the operator is, human error is an inevitable variable.

Nowadays, we use the term "tickling the dragon's tail" metaphorically to describe any situation where someone is playing with a force they can't fully control—whether that's AI development, gain-of-function research, or high-frequency trading algorithms. The dragon is always there. It’s usually asleep, but it’s a very light sleeper.

Actionable Insights for Understanding Nuclear History

• Study the Fermi-level physics: If you want to understand why the core reacted the way it did, look up "Neutron Reflectors." The beryllium and tungsten carbide weren't just "covers"; they were mirrors for subatomic particles.
• Acknowledge the Human Element: Recognize that the early atomic age was driven by a mix of extreme brilliance and almost unthinkable recklessness. Reading the original Los Alamos reports (many are now declassified) reveals a startling lack of formal safety protocols.
• Differentiate Radiation Types: Understand that Slotin was killed primarily by neutron radiation, which is far more damaging to biological tissue than the alpha or beta particles most people associate with "radioactivity."
• Respect the Margin: In any technical field, the "margin of safety" is the space between a functioning system and a catastrophe. The Slotin incident remains the definitive case study in why that margin must be mechanical, not manual.

The story of the Demon Core is a reminder that the most dangerous component in any complex system is often the person holding the screwdriver.