Prompt Criticality
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Prompt criticality is all about the neutron lifecycle. In a reactor, neutrons have a certain likelihood to interact with matter and create new neutrons before slowing down or being lost. This process (fission) creates heat. There are 2 types of neutrons that contribute to the neutron lifestyle, fast neutrons and slow neutrons. In a critical reactor the total amount of neutrons created to produce a certain amount of heat creates the same number of neutrons each cycle. Slow neutrons are great in the fact that that we can see their effect on the neutron lifecycle and allow more of them to interact to create heat (supercritical), or allow less of them to contribute to produce less heat (subcritical) when these reactions stabilize and we create a new stable temperature we are critical again. But slow neutrons are just a percentage of the total neutrons in the lifecycle.
Imagine that in a lifecycle 75 are fast and 25 are slow. Those 75 create a bunch of neutrons really fast to that go on to create more neutrons but not as much as 75 and those fast neutrons created interact again and again to create a bit less and less but a while later those slow neutrons interact and make more neutrons some fast and some slow and you end up again at 75 fast and 25 slow. The fast neutrons interact so fast we can’t detect them properly of react to the changes in heat they cause.
Theoretically we could have a reactor that worked only using fast neutrons but it would have to make extremely small changes in reactivity then detect and make changes impossibly fast. Practically if you had only fast neutrons and you introduced too many you would would end up with an exponential chain reaction. Say you went from 75 and we introduced 25 more and now we have 100 fast neutrons as our source for the next lifecycle. These 100 have just enough to produce 101. But 101 is enough to create 125, and 125 is enough to create 726 and so on, only this is happening in a blink of an eye and with milliseconds and you have enough heat to turn all your liquid coolant to turn instantly into a gas and create an explosion.
An exponential reaction of fast neutrons would make a reactor have prompt criticality.
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