Editing Nuclear fission (section)

===Natural fission chain-reactors on Earth===
[[Natural nuclear fission reactor|Criticality in nature]] is uncommon. At three ore deposits at [[Oklo]] in [[Gabon]], sixteen sites (the so-called [[Oklo Fossil Reactors]]) have been discovered at which self-sustaining nuclear fission took place approximately 2&nbsp;billion years ago. French physicist [[Francis Perrin (physicist)|Francis Perrin]] discovered the Oklo Fossil Reactors in 1972, but it was postulated by [[Paul Kuroda]] in 1956.<ref>{{cite journal |author=P. K. Kuroda|doi=10.1063/1.1743058|title=On the Nuclear Physical Stability of the Uranium Minerals|year=1956|page=781|issue=4|volume=25|journal=The Journal of Chemical Physics|url=http://nuclearplanet.com/Kuroda%201956.pdf|bibcode= 1956JChPh..25..781K}}</ref> Large-scale natural uranium fission chain reactions, moderated by normal water, had occurred far in the past and would not be possible now. This ancient process was able to use normal water as a moderator only because 2&nbsp;billion years before the present, natural uranium was richer in the shorter-lived fissile isotope <sup>235</sup>U (about 3%), than natural uranium available today (which is only 0.7%, and must be enriched to 3% to be usable in light-water reactors).