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
Criticality is a tricky concept to visualize, which is why I created this [little simulator](http://blog.nuclearsecrecy.com/misc/criticality/), which is mostly for thinking about bombs (not reactors) but is still useful, I think.
The way to think about criticality is very, very basic. It’s nothing magical. There are atomic nuclei that can be made to split (fission) when they encounter neutrons. Not all nuclei can do this, and not all neutrons split nuclei equally well (the amount of energy the neutrons have changes the probability that they will split a nuclei — paradoxically, having _less_ energy is better splitting many nuclei).
Neutrons usually only go a few centimeters before they disintegrate. When an nucleus splits, it releases a few neutrons (on average about 2.5) in random directions.
With that in mind, “criticality” means you’ve set up your atoms (and anything else around them) in such a way that on average, the neutrons that are being released from those splitting atoms will go on to split at least the same number of atoms that had split in the first place. So if one atom splits and releases 2 neutrons, you’d need 1 of those to split another atom for it to be “critical.”
“Supercritical” means you are splitting _more_ atoms than were splitting in the first place. So that one atom splits, releases 2 neutrons, and each of those neutrons split two more atoms. That leads to an exponential growth.
What affects criticality? A lot of things. It’s really just, “what factors will make it more likely for those neutrons to split more atoms?” They include:
* How many atoms of the splittable sort are in the material. If there is only one atom to begin with, it’s not going to split any more! But more practically, those neutrons are going to be traveling through space, and if they aren’t traveling through other atoms of the splittable sort, then they won’t likely encounter them. This is why a raw amount of mass is usually associated with this.
* How many atoms are mixed in with them that won’t split, but will absorb neutrons? This is an issue in bombs (it is why you need to enrich uranium, because the most common kind of uranium won’t split easily), and reactors (where it is used for control rods among other things). If your neutrons are being “eaten” by other atoms, that will change the criticality requirements.
* Are the neutrons being slowed down, to increase their likelihood of splitting? In a reactor there are substances like water or hydrogen or carbon (the “moderator”) against which the neutrons can be made to bounce to lose some of their energy, which increases the chance that they will split any splittable atoms they find.
* How close together are the atoms? This matters for bombs more than anything else, but one way to increase the chances of neutrons finding more atoms to split is to physically squish the atoms closer together, which is how an implosion bomb works (it uses explosives to increase the density of the fuel before splitting its atoms).
* What geometry is the fuel material? A solid sphere is the most favorable geometry for criticality, because any given atom in the center is likely to have its neutrons run into more material as it moves outward. Whereas a long tube (cylinder) of material is not very favorable to criticality, because the neutrons would have to be traveling along the length of the tube in order to run into more fuel. Careful choice of geometry makes a big difference in whether systems are likely to be safe or go critical.
There are some other factors as well, like temperature and chemical structure and other things of that nature, but you get the picture. It is really just about whether those neutrons are going to find other atoms to split, and anything that affects that will affect criticality.
I find that when we speak of this only as a “critical mass,” it obscures that we are talking about a “system” with a lot of properties, and can make it seem rather magical. But it’s very physical, and somewhat straightforward if you understand what is going on “under the hood.”
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