The Fermi Chronicles - Part 9: Nuclear Fission

This is where the magic occurs. Nuclear fission. In the last installment, I wrote about neutron scattering. This will be important here. In fission, when a neutron strikes an already unstable atom, most specifically uranium, it gets into such a highly excited state that the nucleus actually splits into two nuclei. This split is called fission. A byproduct of the fission results in the emission of 2 or more neutrons that will collide with other uranium atoms and the chain reaction continues.

The chain reaction is a necessary part of the picture so that the fission reaction is self-sustaining. Since the nuclear force is unimaginably large being that's it's in such a small space, a good chunk of energy is given off in the process of fission. That energy can be harnessed to generate electricity. Or to blow things up.

Now not all elements can be fissioned in this way. There are two types of material that are very important to understand - fissile material and fissionable material. And there a huge difference between them and that difference resides in the behavior of neutron. I didn't get into this before, but there are 2 types of neutrons - fast neutrons and thermal neutrons. The difference between the two is speed. Fast neutrons come right out of the fission reaction. If they start hitting other things, though, they slow down and become what are known as thermal neutrons. Thermal neutrons aren't powerful enough to fission some materials, but can fission other. This will become extremely important in nuclear reactors that we harness for power.

Fissile material: this is material that is susceptible to fission by any neutrons whatsoever - fast or thermal. Very few materials are fissile. Examples of fissile materials are U-235, U-233 and Pu-239. And that's about it!

Fissionable material: this material is susceptible to fission by either fast or thermal neutrons. A small subset of fissionable material, is fissile material. Most fissionble material, however, requires fast neutrons for fission to occur. Examples of fissionable material that is not fissile are Th-232, U-238, and Pu-240. These materials are far more common than fissile materials!

The cause of the difference between fissile material and the rest of the fissionable domain is in the critical energy necessary to fission the nucleus. This varies by material. Now, you might think that since we only have a finite supply of fissionable material and that only a small fraction of fissionable material is fissile, then we're screwed. You would be wrong, though. We can create more fuel! But that's for another episode.

Previously:
The Fermi Chronicles - Part 8: Neutron Interaction
The Fermi Chronicles - Part 7: Radioactive Decay and Half-Life
The Fermi Chronicles - Part 6: Atomic Structures
The Fermi Chronicles - Part 5: Nuclear Waste Storage
The Fermi Chronicles - Part 4: Radiation Types and Radiation "Dose"
The Fermi Chronicles - Part 3: Radiation Types
The Fermi Chronicles - Part 2: A week of training
The Fermi Chronicles - Part 1: The alpha post