Prompt Criticality

Question

Prompt Criticality

In: 3

Answer 1

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.