Natural nuclear fission reactors

The primary evidence for the existence of natural nuclear fission processes is the uranium-235 isotope. The main study took place in Oklo, Gabon, in June 1972. A team led by Dr. H.V. Bouzigues from the CEA laboratory in France noted anomalies in the abundance of the uranium-235 isotope. Some time later, much larger depletions of this isotope were discovered in uranium samples from this source. This was the first positive evidence for the hypothesis that natural chain reactions were responsible for the changes in the isotope's abundance. The report issued as a result of this study was:

"It turned out that during the reaction, the natural abundance of the relatively rapidly decaying isotope uranium-235 was more than 3%. This is a natural 'enrichment' due to the presence of water in the soil, which increases the efficiency of partitioning and the possibility of a chain reaction developing."

In summary, we see that natural nuclear reactions occur in nature—even on Earth's surface. Add to this the possibility of a much higher concentration of uranium-235 in the past. Even the small sample at Oklo would have been enough for a nuclear reaction lasting 500,000 years. Does this have anything to do with the possibility of an inner sun? Uranium is a heavy element and could have accumulated at Earth's center... but there is no evidence for this, as scientists claim the element does not occur naturally.