Criticality: “Let’s get something straight…”

Criticality. “It’s gone critical!!” A word and phrase that resonates within many of us to produce thoughts of nuclear warheads, meltdowns, and death. Would you believe me if I told you that criticality, or the ability for a process to go critical, is a wonderful phenomenon of equilibrium that makes all life possible? Well it is, so you’d best believe it.

  • Criticality:  An equilibrium of input versus output; applies to any subject / situation in which this condition is met. (Read: Not an exclusive nuclear process)

Having seen earlier that a fission event produces several different things, here we shall focus on only one of them. The fast neutrons. Fast neutrons are nothing more than neutrons with a whole bunch of kinetic energy.

Explanation of “fast” neutrons: Neutrons have a defined mass, and therefore a defined rest-energy. If they have more energy than their inherent rest-energy, they must be doing more than just sitting there. If they are not sitting there, they are moving. Kinetic energy is the energy of motion. For this reason, when the total energy of a neutron is mostly made of kinetic energy, we say they are “fast.” To learn more about Neutrons and rest energy: Here.

For criticality to be achieved, at least one of those neutrons from a fission event must hit a fuel atom and cause a fission. Not every created neutron will interact with a fuel atom, so it is generally required to produce at least 2 neutrons per fission. Any fuel used in a nuclear reactor must therefore average 2-4 neutrons released per fission to make sure at least one finds its target. To put it more simply; you need to get out at least as many neutrons as you put in. Getting an average one-to-one ratio of neutron input and output is achieving criticality. To do this-and do it well-engineers and scientists spend time doing things like neutronics modeling and working on reactor design, utilizing, among other things, the six-factor formula.

If achieving criticality takes scientific work and engineering, there must be a point above and below the critical state. Thankfully the nuclear industry made them easy to remember! We have sub-critical and super-critical states of being. As you might imagine, sub-critical reactors are are not producing enough neutrons to keep themselves going. Conversely, super-critical reactors are producing too many fuel-interacting neutrons. 

So how do we control the neutrons to achieve criticality? Good Question! In comes the moderator to make sure all those neutrons we created know what to do with themselves.

Neutron moderators are used to control the fluctuations between sub-critical and super-critical, with the overall average ratio of the input/output varying on the fact that some reactors want fast neutrons to continue fission reactions while others prefer slow neutrons. It’s all dependent upon the design type and the fuel’s neutron cross-section. 

What’s important here is reactor design. Reactors can be designed, based on fuels and systems implementation, to deal with criticality in one of two ways. Either they are designed to be inherently super-critical or inherently sub-critical. This means that, if you were to do nothing and walk away from it while in operation, the reactor is designed to either naturally want to get physically hotter (tend to go super-critical) or it tends to get physically colder (go sub-critical) on it’s own.

Newly designed reactors are now being created to be inherantly sub-critical; however, most power reactors in use today tend to go super-critical!

“ Boy, That sure sounds dangerous! Why Alex?”

Decades ago-when these reactors were first invented-they were made this way because it was easier to maintain and sustain the chain reaction without advanced computers and modern robotics. Think about it, these were originally designed in the mid 1900’s. They persist today because these kinds of reactors have the most research and operational experience behind them. This makes them generally safer (one could argue) because we know with almost absolute and abundant certainty what is to be expected. It is the “devil you know” mentality in many ways. In fact, the Nuclear industry is among the safest industries even with these kinds of reactors. But I’ll let you know this-there are new innovations, which utilize modern computing, modern engineering principles, modern materials, and modern robotics, which are game changers to the industry and humanity as a whole. They are called Generation 4 reactors and are capable of ushering in a new era of clean, sustainable, and carbon-negative energy.  More than that, they can bring about a period of prosperity that would make the industrial revolution look like a few kids playing in a sandbox. 

Prior to reading this, what were your thoughts about a nuclear reactor going critical? Has your perception of the nuclear industry changed at all?  Please leave any comments and feedback below.

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-Alex Kernan

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