system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is the demon core and why is it dangerous?

I keep seeing memes about it. From what I gather, it's a metal orb divided into two semi-spheres, and when they touch, something bad happens. I looked at the wikipedia page and understood very little lol.
gpt
So to understand what the demon core is you need a bit of background on nuclear materials:

Plutonium isn't particularly dangerous in and of itself. 

There are four types of radiation: alpha particles, beta particles, gamma rays, and fast neutrons.  Of those, beta particles and fast neutrons are the most dangerous because they can penetrate the outer layer of your skin, but can't penetrate your body.  That means that 100% of the energy they contain gets absorbed by the living parts of your body.  Gamma rays mostly pass through you without being absorbed, so they're only dangerous if there is a lot of them.  Alpha particles can't penetrate the outer layer of your skin, which is made up of dead skin cells, so they're only dangerous if you eat something that's giving them off.

Plutonium is extremely radioactive but the vast majority of its radioactivity is in the form of alpha particles.  That means that its safe to handle plutonium without protective gear, as long you wash your hands off afterwards.

Nuclear bombs will detonate on their own whenever enough nuclear material is placed in a small enough space.  For a uranium bomb, you just need to make a big enough sphere out of uranium.  Uranium is dense enough that it will detonate on its own once you get enough of it together.  Plutonium is different.  Plutonium isn't dense enough to detonate, regardless of how much of it you have.  That being said, it only requires a teeny, tiny increase in density to cause plutonium to detonate.  That means that a plutonium bomb core is always sitting on the verge of detonation.

Fortunately, its pretty hard to increase the density of plutonium, so a plutonium bomb core is not only radiologically safe to handle, its essentially the most insensitive explosive in existence.  But there are certain things you can do to turn a plutonium bomb core into a nuclear reactor without causing it detonate.  Sticking it near beryllium is one of those things.

The demon core was a plutonium sphere that had originally been intended to be used as the core of a nuclear bomb.  Because it was safe to handle, it was repurposed as a sort of training aid following World War II.  One of the things they were training themselves on was in assembling and disassembling simple nuclear reactors - that is, putting as much beryllium as they could around the core without turning it into a nuclear reactor.  

Two different people screwed that up on two separate occasions, both times briefly turning the core into a nuclear reactor and irradiating themselves with fast neutrons.

That might sound like a dumb thing but its *relatively* safe to do - they did it thousands of times with only two accidents.  And its how they made a lot of the early plutonium for the Manhattan project - there were a few open air reactors that were essentially just unrefined uranium that had been stuck in between sheets of graphite.  When you stick uranium in between graphite like that, it becomes a low power nuclear reactor that doesn't produce much energy but does produce a lot of plutonium.  In the early Manhattan Project they were more or less doing that by hand.  Sticking uranium into graphite is the same basic concept as sticking plutonium into beryllium, albeit somewhat safer because of the lower amount of energy generated.

When you see pictures of the demon core, what you're typically seeing are pictures of the beryllium plates that they were putting around it.  The core itself was a solid sphere.