What exactly was the demon core and what did it do?

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I’ve read the Wikipedia article and watched a couple videos I’m still a bit confused

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It was a sphere of plutonium. Nearly large enough to go critical, it was used in a number of experiments to understand criticality in a nuclear device. Because of the technology of the age, some of those experiments were conducted by scientists. In some of the experiments unexpected events caused huge releases of nuclear radiation and a number of scientists were killed.

So, the important things about plutonium and other fission fuels (thorium, uranium-235 etc) are

– exposing them to free neutrons makes them unstable
– and when they decay they *produce* free neutrons

So they’re the nuclear equivalent of being on fire, very slowly smoldering unless you do one of two things:

– add neutrons from an external source
– do things that help the neutrons stay inside the fuel

Scientists were experimenting with the second technique, called “criticality.” They made a piece of plutonium that was very close to the critical point. Normally, enough neutrons (barely enough) were able to escape into the environment (hopefully into a shield) but a nearby neutron reflector would allow the reaction to grow.

It grows slowly at first but keeps accelerating (exponential growth) and after that it still keeps accelerating, and keeps accelerating until something changes and causes neutrons to escape.

This is similar to a fire with a *lot* of available fuel. Like a fire in a lumber yard. It can smoulder for a while but when it starts to become big, it becomes BIG shockingly fast.

Both accidents involved putting pieces of reflector material near the core, by hand and fumbling it somehow. The flare-up (a “power excursion”) released a lot of heat and radiation.

Fun fact: experiments to find the critical point are *still* very common. That’s how you start up a power reactor – gently find the critical point. You can guess pretty well by using the data you have, but the exact point depends on the exact condition of the fuel in the reactor.

The difference is that there are fast and automatic shutdown systems, the intended startup rate is pretty slow, all the controls are remote, and of course there’s a *lot more shielding.* Nobody starts chain reactions by hand anymore. Especially not while wearing cowboy boots.

The way nuclear fission works is you take a large, unstable element, like Plutonium and shoot a neutron at it. This causes it to split apart and release a lot of energy, and lots of other neutrons.

If there is more Plutonium near by, then those neutrons might hit that Plutonium as well and cause them to split apart and release even more energy.

If you have enough Plutonium in a confined enough space, this starts a self-sustaining chain reaction. This amount is known as the *critical mass.*

To prevent this run away chain reaction from happening you can either just not have a critical amount of Plutonium in that space, or you can separate it. Remember, it’s not just about the amount, but about having that amount in a small enough space (e.g. having enough Plutonium atoms close enough together that they’ll get hit by all those neutrons flying around).

The Demon Core was two half-spheres of Plutonium that, together, were critical, but were kept physically apart as to not be critical.

But the manner in which it was kept apart was rather haphazard, relying on people just being careful which, of course, they weren’t, and there were several causes of the two halves touching or coming close enough to make the entire thing critical, temporarily starting a chain reaction, which produces a lot of radiation.