See this big pile of rocks? It's actually a fully functional nuclear reactor. Or, it was, about two billion years ago.
All that a nuclear reactor is, is an area where there's a high enough concentration of uranium 235 (around 3% or so) such that a fission reaction is sustainable. You don't generally find concentrations of the uranium 235 isotope that high without refining uranium ore, since at this point, it's all decayed away. But two billion years ago, when a bunch of uranium got together into one big deposit for the first time, the concentration was apparently high enough to start a chain reaction.
Thanks to the intermittent intervention of groundwater (which acted as a neutron moderator), this chain reaction managed to sustain itself, on and off, for a few hundreds of thousands of years, in sixteen different areas in what is now a uranium mine in Africa. The above pic shows what's left of one of these reactors, which at this point, is just some uranium oxide with traces of things like xenon, which are byproducts of fission reactions.
Oh, and for the record, 100 kilowatts is really not that much electricity; it's about enough to run a few dozen toasters, if you're a big fan of toast. But still, that's not bad for a pile of rocks, right?