Nuclear satellites and probes use tiny reactors only capable of watts of output.
THAT would be cool. The reality is a little more boring but a lot more safe and practical. Radioisotope thermoelectric generators are nothing more than a high-efficiency version of those pots that charge a cell phone from the heat of your camp fire. They use what most here would recognize as Peltier coolers, though optimized for operation in reverse. (generating electricity from a heat differential.) There is no nuclear reaction taking place, only the natural decay of radioisotopes. And that, only for a fairly low-level heat source.The devices could work off any heat source. It doesn't even take much, given the differential created against the cold of space. Certain radioisotopes just happen to do so reasonably consistently for the length of time the missions require. (Heat output declines as the material decays.)
Similar to the camp fire pots, they require a large temperature differential relative to the energy they produce. You're not going to power a colony off of them, at least not one that is reasonably self-sufficient, complete with manufacturing capability.