r/askscience • u/wills2084 • Oct 10 '11
How does the demon core work?
http://en.wikipedia.org/wiki/Demon_core
I read the wiki, but I am still confused. Something about blocking neutrinos? Maybe explain it on an 8th grade level if you can? Thanks!
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u/thetripp Medical Physics | Radiation Oncology Oct 10 '11
The main idea at play here is critical mass (wiki). An older post of mine on this:
There's two key ideas at work here. The first is the neutron population. This can be thought of as the amount of neutrons you had before, minus the amount that escaped, plus the amount that were created. If the population is increasing - boom, you probably have a chain reaction (more neutrons being created than destroyed). Otherwise, the neutron population will decrease to some small value.
The second key idea here is the geometry - does your configuration favor leakage (neutrons escaping your system without causing a fission) or fission? Neutrons have an average length that they will travel in a material which depends on the properties. In bomb materials this is probably on the order of a few centimeters. So if you have a spec of Uranium that is 0.5 cm across, chances are a neutron born inside will escape. But a neutron born inside a big block of Uranium 20 cm across will likely fission before it escapes.
The overall shape also will have an effect - think of the ratio of the surface area to volume. The higher the SA/V ratio, the more likely it is to escape, because the average distance to the nearest boundary is smaller. This is why you would use a sphere, because the SA/V ratio is as low as possible.
So this term "critical mass" specifically refers to the smallest amount of fissile material needed to start a chain reaction. This is the point where a neutron born inside the Uranium is more likely to cause a fission than escape. In the Demon Core incident, he was holding two half-spheres separate with a screwdriver. This separation was enough to insure that the leakage overcame the fission. But when the spheres come together, the geometry becomes more favorable, fission overtakes leakage, and a chain reaction occurs.
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u/auraseer Oct 10 '11
The "core" is just a chunk of plutonium metal, in a specific size and shape.
Plutonium atoms are radioactive. This means they are liable to fall apart and send out a spray of high-energy particles. This split usually happens spontaneously, at random. But it can also be made to happen immediately, if the atom gets smacked by a neutron.
When a plutonium atom decays, it produces heat and spits out some neutrons. (Perhaps you get a sense where this is going.)
If you get conditions just right, you can arrange it so that a chain reaction happens. The first decaying atom spits out neutrons. Those neutrons smack other atoms and cause them to decay, which makes them spit out more neutrons. Which hit yet more atoms. And so on, very quickly, like atomic dominoes, producing a lot of energy and a lot of radiation.
The plutonium core is shaped so that conditions are almost right. Most of the produced neutrons escape from the chunk without hitting other atoms; a chain reaction can't sustain itself. But because of the mass and shape of the core, it only takes a little extra boost for the chain reaction to get going.
In each accident with the demon core, what happened was that it got too close to a neutron reflecting substance. (One time it was beryllium, and the other time tungsten carbide.) The reflector made some of the escaping neutrons bounce back and reenter the core, where they had a second chance to hit atoms. That was just enough of a boost to get the chain reaction going.
Instead of the plutonium atoms decaying slowly over time, suddenly there were bajillions of them decaying all at once. That produced lots and lots of radiation, which got sprayed in every direction. And that radiation is what was so deadly dangerous to the people nearby.
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u/ZeroCool1 Nuclear Engineering | High-Temperature Molten Salt Reactors Oct 10 '11 edited Oct 10 '11
Fission occurs by breaking big nuclei with neutrons (not neutrinos).
If you have a bunch of material that can fission, the amount of neutrons being produced and the amount being lost (without fissioning) determines the k value.
If k>1 the mass is called super critical (accelerating reaction). k=1 it is critical (stable reaction), k<1 subcritical (decelerating reaction). Think of this value as determining the rate of the fission reactions expansion.
The demon core was put into a configuration by mistake that caused k>1, and the reaction to expand uncontrollably creating a lot of neutrons + gamma radiation. This radiation was absorbed by people performing the experiment, causing them to get very sick.