When someone think about nuclear power, their first thought is of big towers with radioactive sign on them, mostly. First, this towers have nothing to do with energy generation, they are part of cooling system and they are appropriately named "cooling towers." It is open-cycle cooling system which using evaporation to cool water. It is cheap and maintenance-light if you have reliable water supply and so, it is used on most powerplants that use coal, gas, nuclear reactor or other source of heat.

To actual nuclear generators: there are two types. Theromoelectric and reactors.
Reactors can be fission reactors, which are used on all atomic energy plants today, and fusion reactors, which are theorised but haven't been made yet, as fusion bombs don't count.

Fission reactors utilise power of radioactive nuclear fission, which means that heavy atoms split into lighter atoms, usually emmiting stray electrons, alpha-particles (helium ions) and gamma-radiation. In theory, fission can be induced, but the lighter the atoms, the less energy gain until we reach iron, where further fission will result in energy loss. You can find them in RimAtomics mod.

Fusion reactors utilise power of radioactice nuclear fusion, which means that light atoms split fuse into heavier atoms, usually emitting stray electrons, beta-particles (lone electrons) and gamma-radiation. Again, fusion further than iron results in energy loss, which is why iron is so abudant: it is the most stable element. Fusion of hydrogen and helium is very energy-profitable and clean, but starting fusion reaction require vast amount of energy, which is why gas giants do not spontaneously ignite into new stars. Engineers of whole world are trying to solve this problem, as this will give us a lot of cheap and clean energy.

Radioisotope Thermoelectric Generators, or RTGs for short, utilise power of radioactive decay, naturally occuring process in unstable elements, such as uranium or plutonium. They last years without maintenance, but give little power. They are usually used as power sources for autonomous polar bases, deep space probes and as heat source for mars rovers. This mod adds them.