Eli5: What is nuclear criticality?

Question

I’ve been binge watching YouTube about nuclear criticality events, and I get the basic principle that you have two radioactive items and they come in contact long enough to create an event.

But I am not quite getting it. So for example, I watched a video about the demon core, but I am not sure how the shims were preventing criticality.

Thank you.

In: Physics

Answer 1

Criticality is the point where 10 year old you and your 10 year old friend lit a string of firecrackers in the hayloft of farmer Mcgregor’s barn, and you couldn’t run down the ladder fast enough to fetch a bucket of water from the well, and the whole barn went up in one giant fire.