https://en.wikipedia.org/wiki/Natural_nuclear_fission_reactor#Mechanism https://en.wikipedia.org/wiki/Natural_nuclear_fission_reacto... says: > A key factor th

https://en.wikipedia.org/wiki/Natural_nuclear_fission_reactor#Mechanism https://en.wikipedia.org/wiki/Natural_nuclear_fission_reacto... says: > A key factor that made the reaction possible was that [back then] the fissile isotope ²³⁵U made up about 3.1% of the natural uranium, which is comparable to the amount used in some of today's reactors.