Nuclear bomb exploding. Scientists in the foreground.
Sarah Biren
Sarah Biren
June 1, 2024 ·  4 min read

Objected Dubbed ‘The Most Dangerous Ever’ Was Created By Accident and Killed Its Inventors

The film “Oppenheimer” has brought widespread attention to the creation of the world’s first atomic bomb. It involved a team of researchers and scientists called the Manhattan Project during World War II. But after the war, the U.S. scientists used a leftover spherical piece of plutonium to figure out “the tipping point” that would lead to nuclear explosions — also called going “supercritical.” This dangerous object has been dubbed the “demon core” after it caused horrific deaths to members of the Manhattan Project.

The Most Dangerous Object Ever Made

A recreation of the dangerous demon core
Photo: Los Alamos National Laboratory

Now, the demon core was not used in warfare. It was meant to be the third bomb strike on Japan after the U.S. detonated atomic bombs called ‘Little Boy’ and ‘Fat Man’ over Hiroshima and Nagasaki, respectively. These two attacks killed hundreds of thousands of citizens before Japan surrendered on August 15.

So, the scientists of the Manhattan Project were left with a functional core of an undetonated bomb in Los Alamos. They began to study the leftover nuke, and during these dangerous tests, the object earned the name the demon core.

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Tickling the Dragon’s Tail”

A recreation of 1945 accident that killed physicist Harry Daghlian
Photo: Los Alamos National Laboratory

The scientists at the Los Alamos Lab set out to determine how far the core could be probed until it entered supercritical mode. They were all aware of how dangerous it was to conduct criticality experiments on the object. They even had a nickname for the perilous task: “tickling the dragon’s tail.”

The Death of Harry Daghlin

Harry Daghlian’s blistered hand after an accident with the demon core
Photo: Los Alamos National Laboratory

On August 21, 1945, Los Alamos physicist Harry Daghlian returned to the lab after dinner. Everyone else had gone home aside from the security guard. Daghlian resumed experimenting on his own, which was against safety protocol. 

During this test, Daghlian began building a tungsten carbide brick wall around the demon core. These bricks reflected the core’s neutron emissions back at it, slowly tipping it toward criticality. Daghlian continued to build the wall around the core until his monitoring equipment showed that the plutonium would go supercritical if he added any more tungsten carbide.

But when Daghlian tried to dismantle the wall, he accidentally dropped a brick onto the core, which went supercritical and sent out a flash of heat and blue light. Daghlian instinctively reached out to remove the brick and felt a strange tingling in his hand, which blistered over. He had been exposed to a fatal dose of radiation and died 25 days after the incident after weeks of pain, nausea, and a coma. The security guard on duty was killed as well.

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The Death of Louis Slotin

A recreation of the 1946 accident that killed physicist Louis Slotin
Photo: Los Alamos National Laboratory

Despite the tragic deaths, no security measures were taken to prevent something like that from happening again. So on May 21, 1946, physicist Louis Slotin was demonstrating a similar experiment, this time by lowering a dome made of beryllium over the demon core. Slotin used a screwdriver to keep a gap between the dome and the core, allowing enough neutrons to escape and keeping the demon core stable.

However, the screwdriver slipped, and the dome fell over the core, which was fully covered. The lab filled with a flash of blue light and heat for the second time within a year. Slotin quickly pulled the cover off but there was no changing that moment. He, five other scientists, a photographer, and a security guard all received high exposure to radiation. However, Slotin was the only one killed by it after nine days of nausea, vomiting, pain, and mental confusion.

The End of the Most Dangerous Object Ever Made

A recreation of how the demon core might have appeared during Slotin’s demonstration
Photo: Los Alamos National Laboratory

After two lethal accidents, new protocols came to end “hands on” criticality experiments, requiring scientists to use remote control machinery while they are a safe distance away. Additionally, the demon core was intended to be used in Operation Crossroads, the first post-war nuclear explosion demonstration. However, that plan was nixed after what had happened.

So, the plutonium was melted down into a nuclear stockpile, never to be detonated as the bomb it was created to be. The deaths of Daghlian and Slotin represent the many fatalities from the end of World War II. In more ways than one, the demon core was the most dangerous object ever made.

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  1. “The Demon Core and the Strange Death of Louis Slotin.” The New Yorker. Alex Wellerstein. May 21, 2016
  2. “The Chilling Story of The ‘Demon Core’ And The Scientists Who Became Its Victims.” Science Alert. Peter Dockrill. April 3, 2021
  3. “The Demon Core: A Tale of Atomic Ambition and Tragic Fate.How Stuff Works. Clarissa Mitton. April 3, 2024