So this isn't going to be like your typical Weekend Science episode. I'll go over a small glossary and explanation so you can follow along, but this is going to be a story. A true story, mind you, not a fictional scary story you toss back and forth around the campfire but I find it quite terrifying. This is the story of a 6.2 kilogram sphere of Plutonium that accidentally achieved criticality- a sustained nuclear chain reaction- twice in the lab at Los Alamos, once in '45 and once in '46, killing a scientist working on it each time. Then we let it go critical one last time, as the nuclear bomb Gilda detonated as Test ABLE in Operation Crossroads and unleashing a new, more powerful method of building 'the bomb' then the already demonstrated gun-type atomic bomb.
(If you have the basics of nuclear reactions and weapons down, feel free to skip down to after the first block quote, thats where the story starts)
First the background: Nuclear bombs and nuclear fission is, in broad strokes of explanation, a simple thing to explain. Radioactive particles break apart, free neutrons hit other atoms, breaking them apart, resulting in a chain reaction known as 'criticality'. Prompt critical, which the demon core achieved both times in the lab, and three times total, is when neutrons immediately hit and release more neutrons in collisions (hence the 'prompt'), resulting in exponentially expanding chain reactions. Slower chain reactions are how nuclear power plants work. The uncontrolled variety, usually with substances added for more neutrons (commonly Tritium, a form of water with extra neutrons) and neutron reflectors are how nuclear bombs work.
There isn't much chance for Uranium or Plutonium to just blow. You can pick it up and be fine. Even Depleted Uranium bullets are harmless radioactivity-wise (it is, however, a toxic metal). Their rate of decay is measured in tens of thousands to millions of years, not enough strong enough over the time of your entire lifetime for decay particles to fry you. Depleted Uranium is used to protect you during Radiation Therapy- it's used as a radiation shield because of its density. Plutonium is the same. Something has to help it along because as dense as it is, it isn't dense enough to naturally sustain a chain reaction; just not enough atoms for the naturally decaying particles to hit with the neutrons they throw off. The scientists working on the Manhattan Project solved it the simple way first: A Gun-type nuke. This was what we eventually dropped on Hiroshima, chosen mostly just to get rid of it as we had already created much more effective nuclear armaments (like the one dropped on Nagasaki several days later).
A gun-type takes U-235, a rare form of Uranium chemically identical to the more common (and not fissionable) U-238, and fires one mass down what is essentially a gun barrel into another mass of U-235, and the compression of the hit and the amount of material in the combined mass is super-critical. The method is so fool proof and easily understood that we never even tested it before deploying. It's as close as you can get to Elementary Science in nuclear physics- my high school physics text book described how to make it. If Osama bin Laden can get to wikipedia, which has multiple diagrams showing how it operates and what goes where and even the best shape for the 'bullet' and 'target, well then he can build one of these. Thats one of the reasons we control nuclear material so tightly- the hard part isn't knowing how to build the bomb, it's getting the parts. Uranium-235 is .7% of any given hunk of Uranium, and can't be chemically separated. That's why centrifuges and gaseous diffusion plants get front page news when discussing Iran, they're how you physically separate the two forms. Gun-types don't even exist anymore, their explosions, as horrifying as Hiroshima was, are minuscule compared to more advanced ways of achieving criticality:
There are currently no known gun-type weapons in existence: advanced nuclear weapon states tended to abandon the design in favor of the implosion type weapon, and new nuclear weapon states tend to develop implosion-type weapons only.
And criticality is where we join the demon core. Following WWII the Manhattan Project continued in it's studies, doing research into criticality and the study of the elements that had captured our attention. Harry K. Daghlian Jr was pushing a core of Plutonium right to the edge. He wanted to pin down the moment of criticality using not increased mass, but neutron reflectors. By constantly bouncing neutrons back through the core and not letting them escape, you could drastically reduce the mass needed to achieve criticality. He slowly built a 'house' of Tungstun carbide bricks around the core:
A re-enactment from the investigation into the accidents, the core partially surrounded by Tungstun carbide blocks.
What happened next could easily come out of a mad scientist movie from the 50's:
In the experiment, Daghlian was attempting to build a neutron reflector by manually stacking a series of 4.4 kg tungsten carbide bricks in incremental fashion around the plutonium core. The purpose of the neutron reflector was to reduce the mass required for the plutonium core to attain criticality.
As he was moving the final brick over the assembly, neutron counters alerted Daghlian to the fact that the addition of this brick would render the system supercritical. As he withdrew his hand, he accidentally dropped the brick onto the center of the assembly. Since the assembly was nearly in the critical state, the accidental addition of the last brick caused the reaction to go immediately into the prompt critical region of supercritical behavior. This resulted in a power excursion.
Daghlian panicked immediately after dropping the brick and attempted to knock off the brick without success; he was forced to partially disassemble the tungsten-carbide pile to halt the reaction. Daghlian was estimated to have received a dose of 510 rems (5.1 Sv) of neutron radiation, from a yield of 1016 fissions and died twenty-eight days later, from acute radiation sickness.
Louis Slotin was next up. Louis was a Canadian physicist working for the American government as part of the Manhattan Project. He was studying criticality using the exact same core Daghlian used while also studying criticality. He used two hollow Beryllium spheres acting as neutron reflectors, separating them with a screwdriver jammed between them and his thumb hooked through a hole in the top. Play-by-play provided by wikipedia:
Slotin grasped the upper beryllium hemisphere with his left hand through a thumb hole at the top while he maintained the separation of the half-spheres using the blade of a screwdriver with his right hand, having removed the shims normally used.
At 3:20 p.m., the screwdriver slipped and the upper beryllium hemisphere fell, causing a "prompt critical" reaction and a burst of hard radiation. At the time, the scientists in the room observed the "blue glow" of air ionization and felt a "heat wave". In addition, Slotin experienced a sour taste in his mouth and an intense burning sensation in his left hand. Slotin instinctively jerked his left hand upward, lifting the upper beryllium hemisphere and dropping it to the floor, ending the reaction. However, he had already been exposed to a lethal dose (around 2100 rems, or 21 Sv) of neutron and gamma radiation.
"As soon as Slotin left the building, he vomited, a common reaction from exposure to extremely intense ionizing radiation" recorded Dr Thomas D. Brock. Slotin's colleagues rushed him to the hospital, but irreversible damage had already been done. His parents were informed of their son's inevitable death and a number of volunteers donated blood for transfusions, but the efforts proved futile. Louis Slotin died nine days later on May 30, in the presence of his parents. He was buried in Winnipeg on June 2, 1946.
At first, the incident was classified and not made known even within the laboratory; Robert Oppenheimer and other colleagues later reported severe emotional distress at having to carry on with normal work and social activities while they secretly knew that their colleague lay dying.
Slotin had been friends with Daghlian, and had spent nights by his bed while his friend slowly died from radiation sickness in the hospital, a sad sort of irony to the whole affair. Slotin's experiment was scheduled to be a final proof of the cores ability to go critical before it was detonated as part of Operation Crossroads. The experiments resulted in a increase of efficiency in the eventual explosion, and the demon core entered not only the fears and nightmares of the people around it, but also expanded are knowledge of the materials that redefined and reshaped the 20th century.