Prompt Criticality


Prompt Criticality

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This just refers to a fission reaction which reaches criticality without delayed neutrons (neutrons from fission byproducts), only prompt neutrons. The result is the most rapid rise in energy possible in a fission reaction.

Prompt neutrons are those produced directly from fission; delayed neutrons are those produced from other sources, such as decay of fission products, etc.

Prompt neutrons, as the name implies, are produced milliseconds after fission and normally make up a fraction of the total neutron population available to cause more fissions. Delayed neutrons are produced later and make up the vast majority of the neutron population. Under normal circumstances, the reactor does not have enough neutrons to sustain a nuclear chain reaction without delayed neutrons. It is this delay that allows the fission process to be controllable.

When a huge amount of positive reactivity is injected into the core suddenly, such as a control rod ejection, the number of prompt neutrons produced may be enough for the core to be critical on prompt neutrons alone (“prompt criticality”). This leads to an exponential rise in reactor power in milliseconds.

In a critical nuclear reaction, each fission event releases enough neutrons to trigger 1 additional fission event.

But the neutrons are released in two ways.

* *Prompt* neutrons are released almost immediately (within ~10^-14 seconds or so). This is the vast majority of the neutrons released.
* *Delayed* neutrons are released later, as part of decays of the products of the initial fission, over the course of a much larger fraction of a second.

If the prompt neutrons alone trigger at least 1 additional fusion event, the situation is *prompt critical*. Every 10^-14 seconds, you multiply the number of decays by some number r > 1. If this were to continue for 1 second, you would have r^(10^(14)) fission events – far more than the number of particles in your reactor or bomb in the first place. In other words, this *can’t* continue for 1 second. It ends as the enormous energy of this rapid decay ramps energy up to the point that the critical mass is blasted apart, ending the chain reaction in some degree of explosion (either a brief flash in the case of a loosely-connected assembly, or a full nuclear blast in the case of a well-compassed mass in a bomb).

All of this happens so quickly that the time it would take to detect it is already too late. 10^-14 seconds is only enough time for light to travel a micrometer or so. The light from this event wouldn’t reach you in time to stop it even if you were standing right next to the pile (or indeed, even if you had your eye pressed up against it).

But if the *prompt* neutrons aren’t enough to do this, you have to wait for the *delayed* neutrons. If those are released after, say, 1 millisecond, you only scale your power output by r^1000 per second, which if r is very close to 1, is not that large a number (e.g. if r = 1.01, it’s only a factor of about 21,000, and in real reactors r stays closer to 1 than that). It now takes a matter of seconds or minutes for the rate to grow out of control, and that’s enough time for detection and measures to tone the reaction back down a bit.

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.