How bricks, a screwdriver, and a 'demon core' of radioactive material killed 2 Manhattan Project scientists

The atomic bombs had been dropped.

But at the Los Alamos lab in New Mexico, scientists working on the Manhattan Project, the top-secret program led by J. Robert Oppenheimer that created the first nuclear weapons, were still experimenting with radioactive elements.

As the war ended, a pair of incidents would underscore the dangers of the atomic age. Two physicists would both die in nearly identical ways after toying with a hunk of plutonium later dubbed an ominous nickname: the "demon core."

The scientists — Harry Daghlian and Louis Slotin — were among those experimenting on the spherical 14-pound mass of plutonium, which wouldn't start a dangerous nuclear chain reaction on its own.

The physicists were trying to push the core as close as possible to the edge before it went "critical," a weaker version of the same kind of reaction that occurred in the atomic bombs.

In both cases, all it took was a simple slip of their hand for things to go horribly wrong.

On August 21, 1945 less than two weeks after the bombings of Hiroshima and Nagasaki, 24-year-old physicist Harry Daghlian was conducting a criticality experiment on the demon core at the Los Alamos, New Mexico testing site, according to The Atomic Heritage Foundation.

Daghlian — who had graduated from MIT at just 17 years old — was using tungsten carbide bricks to build a barrier around the plutonium demon core, according to the AHF. The bricks would reflect neutrons shooting away from the radioactive plutonium back into the core. Daghlian was working to make a "neutron reflector," according to the AHF, which would push the core closer to going critical and reduce the mass needed to start a chain reaction.

The physicist had built four layers of bricks and was about to place a brick over the center when his monitoring device warned that laying the piece would trigger the core, according to The Atomic Heritage Foundation. So, Daghlian started to pull away, but he accidentally dropped the brick directly on top of the core, the foundation explained on its website.

The brick falling into place caused a flash of light, a wave of heat, and a blast of extreme radiation, according to United Energy Workers Healthcare. Daghlian was able to knock the brick off the house, but his fate was already sealed. In seconds, the "demon core" of plutonium core had bathed him in a lethal dose of radiation.

He died slowly of radiation poisoning 25 days later, according to the AHF.

But scientists continued experiments on the core — and within a year, another researcher would be dead.

35-year-old Canadian physicist Louis Slotin was known as a bit of a cowboy on the research project who would make risky moves, according to Canada's History. In May 1946, Slotin was conducting more criticality experiments, this time trying to get even closer to the edge.

Instead of laying bricks around the core as Daghlian had done, Slotin had the plutonium surrounded by two beryllium-coated metal half-spheres, according to the AHF. He was trying to lower the top sphere onto the bottom sphere, leaving as little space as possible between the two so that the plutonium core inside would get as close to critical as possible, the AHF explains on its website.

If the spheres fully closed around the core it would start a chain reaction that would again release deadly levels of radiation. Slotin would wedge a screwdriver between the two spheres to create just enough space to keep them from closing around the plutonium core completely, according to the BBC.

The process became known as "tickling the sleeping dragon's tail," according to a national security report from Johns Hopkins' Applied Physics Laboratory. It was a dangerous experiment, one Slotin had conducted many times before, the BBC reported.

But colleagues warned it could be deadly. Enrico Fermi reportedly told Slotin and other researchers that they would be "dead within a year" if they kept the experiments up, according to the report by Johns Hopkins' Applied Physics Laboratory.

On May 21, 1946, Slotin was working one of his final days on the project, training a new scientist, Alvin Graves, to be his replacement, according to the BBC. He again attempted to experiment on the demon core, sliding the screwdriver between the metal halves.

But, as Graves and six other colleagues looked on, Slotin's hands slipped and the metal spheres snapped shut around the core, sending it to its critical point. Another blue flash of light appeared along with a blast of heat, according to the AHF.

Slotin was able to quickly remove the top sphere, but once again it was too late, the BBC reported. The others in the room survived, but Slotin was not so lucky.

"Well, that does it," he said at the time.

Over the next 9 days, according to the BBC, he died a slow, painful death from radiation poisoning, which causes a complete breakdown of bodily functions — including diarrhea, vomiting, blistering, dizziness, and ultimately, organ failure.

The two deadly incidents earned the plutonium core the nickname "the demon core." Now still radioactive enough to be hot, the core was allowed to cool. It had been scheduled to be used in a nuclear test, but the test was canceled, according to the New Yorker.

That summer, the core was melted down and recycled into new cores for testing.

Los Alamos never allowed its scientists to go hands-on with critical testing again.