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
Imagine a giant pile of firecrackers which can randomly pop, so there’s at least a few pooping every second no matter what.
Imagine there’s a chance of a firecracker which pops makes another pop due to the small burst of heat.
If you surround the pile with something that reflects the shock waves and heat from popping back into the pile, you could make it so a single firecracker starts an increasing rate of popping, so the whole pile pops in a second. That’s like supercritical.
If you somehow cause a bunch of firecrackers in the pile to pop at the same time, it might blow the pile (supercritical). Or it might die back down to the normal rate (subcritical). Or, if you get it just right, you keep a constant high rate of popping (critical).
Obviously, staying right at critical is super hard, especially since for reactors the time scale for these changes is millionths of a second. Reactors wouldn’t be controllable if it weren’t for a special case — some firecrackers have a delayed pop, by up to a few minutes, i.e. they’re hit by a nearby cracker and you know they’re gonna pop, but a few minutes from now.
So you keep the pile in a condition where the “prompt” condition is subcritical, but the “delayed” condition can be sub, super, or critical. You listen to the rate of pops and very slowly change things to raise or lower the rate of popping. But be careful! If you want to raise the rate and you get impatient, you may change conditions so.much that it goes supercritical and the whole pile pops before you can react.
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