Fission: The Origin of the Heat

      Previously, we realized that a nuclear reactor produces heat. Now we shall discuss how Fission produces the heat. First, we need a little bit more Fissionknowledge about Neutrons and to clear up some terminology that is often confusing (because nuclear engineers and Particle Physicists are the worst at naming things!). Fear not, you can learn this in 5 minutes or less.

Neutrons are medium-large particles that do not have charge, but they do have energy and they can have a lot of it!  They have energy because it is both moving (called: “Kinetic Energy”) and is made up of “stuff” (called: “mass”). We learned previously that mass can be thought of as energy too. The energy from the neutron is added to the nucleus by a collision where they basically get stuck together. This is called the “Theory of the Compound Nucleus.” Now the nucleus has an extra neutron, as well as, a whole bunch of extra energy. 

Note: For clarification between energy, charge, and electricity, please click Here


     Fission occurs when the total energy of the nucleus is greater than the nuclear binding energy.

Qualitative Terminology:

  • Total Energy: The sum of the neutrons energy and the target nucleus’ energy.
  • Nuclear Binding Energy: The maximum amount of energy the nucleus can have before it wants to Fission.
  • Fission:  The action of dividing or splitting something into two or more parts.
  • Fissionable: A  material that can fission, but needs a lot of energy “put in” to do it.
  • Fissile: A material  that needs very little energy to fission.
  • Energy Density: The amount of energy expelled from fission for a given amount of substance.”

Now we can talk about the heat that is produced in a nuclear reactor. We have seen that to create a fission event, we need a neutron (“incident neutron”) to impact a nucleus (“target nucleus”) with enough energy that the combined energy is greater than the Nuclear Binding Energy.  When a fission event occurs, several things are released, all of which are special to the type of material that just underwent a fission event. Common products of fission events are: Alpha particles, Beta Particles, Gamma Rays, and a few fast neutrons. For now, we can say that all these particles have energy and are bouncing around, off and through things, while also depositing a little bit of energy along the way. We may simply call this all “heat.” By and large, that’s how we humans can easily identify it. Until now, we have been talking about a single atom and a single fission event. Now, I would like you to imagine this same process happening trillions upon trillions of times. The magnitude of scale brings it all back around to Energy Density described in the previous article,  where a small amount of fissionable/ fissile material can create a very large amount of heat.

Next time we will talk about Criticality (Which is a very good thing! ). If you enjoyed reading about Fission, have questions/comments, or have a topic in mind you would like explained here at, please let us know below.


-Alex Kernan


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