There is a very, very large amount of different reactor types you can build. The main decision points are first, do you want to have 'slow'/'thermal' or 'fast'

There is a very, very large amount of different reactor types you can build. The main decision points are first, do you want to have 'slow'/'thermal' or 'fast' neutrons. If you want to have 'slow' neutrons you need some material to slow them down. This both has advantages and disadvantages. Then you have to decide what the primary material is you want to use to fuel the reactors, here the options are basically U-238 (natural uranium), U-235 or Thorium. Then you have to decide, if you are going to slow neutrons down, there are a few options on how to do that. light Water, PWRs are an example, Heavy Water, like CANDU reactors. Other common options are graphite (Molten Salt Reactor Experiment). But there is also more exotic stuff like beryllium. Once you made that choice you need to somehow cool your reactor. For that you have tons of options all with advantage and disadvantages. Water is the common example but water has to be under a huge amount of pressure for this to work. There are also salt cooled reactor (like fluride salt), or lead cooled, sodium cooled, gas cooled and so on and so on. Check Figure 10: https://unece.org/sites/default/files/2021-08/Nuclear%20power%20brief_EN_0.pdf https://unece.org/sites/default/files/2021-08/Nuclear%20powe... There are more options of course, the fuel can be liquid or solid. And if its solid it can be shaped in different forms. Nuclear reactors can work so many different ways. If you want to see some interesting examples of current commercial reactors check out for example: - Moltex Energy SSR-W - Terrestrial Energy IMSR Here are lots of companies currently engaged with the Canadian regulator: https://nuclearsafety.gc.ca/eng/reactors/power-plants/pre-licensing-vendor-design-review/index.cfm https://nuclearsafety.gc.ca/eng/reactors/power-plants/pre-li...