The Demon Core. What was it and what exactly happened during the incidents it went supercritical?
In: 15
The demon core was a sphere of plutonium which were split into two half spheres. It was used to research the moment that plutonium goes from subcritical to critical. The plutonium on itself is relatively safe in small amounts. But if you collect too much of it, especially in the presence of neutron reflectors, then the neutrons generated naturally will likely hit more plutonium creating even more neutrons. This is the chain reaction behind nuclear reactors and nuclear bombs. The daemon core was designed to be too little for it to go critical, making it safe. But researchers could stack neutron reflectors around it to make it critical. To prevent this they can separate the two halves a bit making it sub-critical. By adjusting the distance between the two halves very carefully you could get it to be just on the edge of becoming critical and make scientific measurements. These measurements made it possible to model plutonium in nuclear reactors and nuclear bombs making it possible to design these devices.
The demon core got its name after two separate incidents where the techniques separating the two halves failed causing it to go critical with scientists standing in the room. Both incidents were fatal.
It only got that name due to being involved in the two criticality accidents that killed two people, and what happened is fairly simple–people didn’t treat a highly dangerous material with the respect it was due. In one of the incidents, for example, they were using a hand-held screwdriver to hold the two halves of the core apart to keep it under the critical mass while experimenting on it, and the screwdriver slipped and momentarily allowed the core to go critical–it was only the researcher reaching in and pulling the two halves of the core apart by hand that prevented a much worse disaster. However, him being closest to the thing meant he received a fatal dose of hard radiation.
As to what happens when a lump of radioactive material goes critical–material undergoing radioactive decay emits smaller particles as the atoms break up, usually neutrons and alpha particles. Some of these go on to hit other atomic nuclei and cause those to break up in turn, but this is a fairly uncommon event because atomic nuclei are very small and the distances between them comparatively large. However, once you have a critical mass of the material enough of these collisions occur to form a self-sustaining chain reaction–e.g. when one atom breaks up, the pieces flung out will hit other atoms and cause those to break up, which flings out more pieces and breaks up more atoms, etc. This massively increases the energy and radiation output of the material and, when set up properly in an atomic bomb, causes a huge explosion.
The cored was a sphere of plutonium that was intended to be used to make a third nuclear bomb to attack Japan after Nagasaki and Hiroshima. When Jason (edit; oops Japan) surrendered after the second bombing, a third bomb, obviously no longer needed. The core was then used for experiments.
As someone else has stated, one incident (the second one I believe) was when the screwdriver that the scientist doing an experiment was using to separate the reflectors they were manipulator around the core slipped. This allowed the two hemispheres of the reflectors to come together and the core reached critical mass momentarily (one way to reach criticality was to have enough plutonium together, another was to increase the number of neutrons reflected back at it. Another is to increase pressure on the plutonium. Both incidents occurred when reflectors were being manipulated).
The scientist who dropped it instantly flipped the top reflector off, stopping further reaction. However he received a fatal dose of radiation and died of acute radiation poisoning. Because of his position on the room his body protected the others there from getting a larger dose. I think one other got acute radiation sickness but survived to leave hospital. I don’t know how his health was after that.
The other incident was when a reflector brick was accidentally dropped on the core while they were arranging them around the core for experiments. I the scientist quickly removed the brick but got a lethal radiation dose and died of radiation sickness. There was only one other person present, a security guard, who was further away and was unharmed in the incident.
[removed]
Terminology about nuclear reactions is very important here.
Sub-critical: Fission events (atom breaking apart releasing neutrons) create less than one other fission event on average. This means that reaction rate of the material is low and remains low. It is not capable of producing a nuclear detonation.
Critical: Fission events create one other fission event on average. The reaction rate is stable and self sustaining. This means there is an increased reaction rate, but its not increasing or decreasing. It is not capable of producing a nuclear detonation. This is the normal range most nuclear power plants aim around. It generates energy in a controlled manner. They will use the control rods to adjust the rate from sub-critical to critical to supercritical as needed to adjust power output.
Super-critical: Fission events create more than one other fission event on average. The reaction rate is increasing over time, normally in an exponential manner as each fission event creates multiple other events. An appropriately managed supercritical event is what creates a nuclear detonation. Super-critical doesn’t always mean nuclear detonation though.
Media loves to use critical and super-critical as if they are the same thing. Because “critical” sounds scary they will accurately state a reaction was critical, but treat it like it was super-critical.
Even sub-critical can be dangerous depending on the reactivity of the material. Its about how much radiation (and what type) is being output, not whether the reaction is self-sustaining or increasing.
What I don’t quite understand about this (and this is after piles of reading about radiation / radioactivity over the years — I’m fascinated by it), is how are these scientists able to handle materials like a giant sphere of plutonium and not be exposed to radiation?
Even if the material is sub-critical, isn’t it still giving off *some* level of radiation? Is it that the level of radiation is negligible? So the Slotin accident is it that it goes from negligible … negligible … negligible … (screwdriver slips) … overwhelmingly fatal dose … (removes sphere) … back to negligible ?
I’ve always thought that handling a piece of uranium or plutonium would result in some level of exposure that is categorically bad …
It is my understanding that it takes 10 LBS. of bomb grade uranium to create a critical mass. Since uranium is slightly more dense that lead you are talking about a fairly small sphere. This is the size of 5 lbs. of lead each segment is 5 lbs. https://www.lowes.com/pd/JSC-25-lb-Lead-Ingot/50305657
So you probably understand a fission chain reaction.
* large unstable atom decays (usually certain uranium or plutonium isotopes), which produces neutrons (radiation).
* protons hit other large, unstable atoms, which causes them to decay, producing even more neutrons.
* this repeats, and increases in intensity VERY QUICKLY.
to accomplish this, you basically need a certain amount of these unstable atoms very close to each other. A little bit won’t work. But if you keep adding more and more, eventually you’ll reach critical mass, and the chain reaction will begin. You’d better not be nearby, or you’re dead, becaue it creates a LOT of radiation, and may explode bigly.
But if you have a piece of, say, uranium that is *almost* big enough (subcritical), you can still get a chain reaction is you surround it with a material that reflects neutrons. instead of those neutrons escaping, they are bounced back into your chunk of uranium, and that can cause the chain reaction to begin. They call this “reaching criticality”.
So Once upon a time, back in the 60’s they had one of these almost big enough chunks of uranium, a subcritical core. And some dumbasses decided to try that. They built a a round shell out of beryllium, which reflects neutrons. the shell had two halves, and was hollow, and you could fit the piece of uranium inside.
As long as the shell wasn’t completely surrounding the core, the core would remain subcritical, but if you allowed the two halves of the shell to close up around it, it would go critical. So you had to keep the halves slightly separated to avoid a chain reaction.
So these dumbasses decided to fuck around by playing with this ultra-dangerous death trap. they would hold the two halves apart with a screwdriver and basically see how much they could close the shell without it going critical. And they were standing **right next to it!!.**
And like most people who fuck around, they inevitably found out. One day the screwdriver slipped, the shell closed, and they got a massive dose of radiation, and they got sick and some of them died.
And then, a while later, some *different* dumbasses tried the same thing, with the same core, and *ALSO* fucked up and got themselves irradiated.
So that piece of uranium got kind of a bad reputation, and was named **the demon core**.
The end.
Latest Answers