When Uranium is refined, they are enriching the U235 vs U238. The ratio is too low today to sustain any natural reactors, but 1.5 billion years ago this wasnt the case.
Due to somewhat obvious reasons (not a lot of net-new uranium being formed, and the half life is fairly stable), the Uranium concentration is pretty uniform around the world and is something like: uranium-235 0.720% uranium-238 99.275% uranium-234 0.006%
Evaporation and other factors wont change this, it hasnt changed this ratio in the muti-billion years Uranium has been on earth. What does change this is the half life of U235. U235 wont naturally 'accumulate', it doesnt move around in rock to form higher concentrations, it is generally 0.720 everywhere.
The natural reactor ran becaues back then there was simply more U235 (half life 700 million years) vs the half life of U238 (4.5 billion years)
No Hypothetical examples needed. Open pit mines wont do anything 'interesting' unless you can either go back in time, or reverse the U235 Decay and then add water and you will probably get something like what took place at Gabon.
https://en.wikipedia.org/wiki/Natural_nuclear_fission_reacto...