There has never been a natural nuclear reactor,
Oh yes there is - 19 of them in Gabon, Africa, now unfortunately destroyed by mining the Uranium they contained.
The natural nuclear fission reactors in Gabon are unique— to date, no additional natural nuclear reactors have been discovered. Unfortunately for science, the sixteen natural nuclear reactors at Oklo have been destroyed, completely mined out for their rich uranium ore. Scientists only have limited uranium samples (often with sparse field notes) on which to conduct their study of these extraordinary nuclear reactors. In the late 1990s, there was danger that the last natural nuclear reactor at Bangombé would be mined as well. In 1997 scientist Francois Gauthier-Lafaye (and co-authors) wrote a plea to the journal Nature advocating that mining of the Bangombé uranium be stopped. They wrote,
The last known natural fission reactor on Earth is likely to be mined this year. Because these natural reactors are unique, at least one should be preserved for present and future research programs… All the reactors except one are located in the most important uranium deposit of Gabon’s Franceville basin, at Oklo…
