|Fig. 1: Geological conditions in Gabon. (1) Nuclear Reactors, (2) Sandstone, (3) Uranium Ore Layer, (4) Granite (Source: Wikimedia Commons)|
In 1956, P.K. Kuroda theorized the conditions under which a natural uranium deposit could sustain nuclear fission over an extended period of time.  In the 1970s, workers were mining uranium at a site in the Franceville Basin, Gabon, Africa. In 1972, H. Bouzigues was working in a French nuclear fuel processing plant and analyzed a sample from the Franceville Basin Uranium mine. After running a mass spectrometric analyses on the sample Bouzigues noticed an anomaly, the abundance of U-235 was found to be 0.7171; the natural abundance of U-235 is know to be 0.7202.  This unexpectedly low concentration of U-235 suggested that at some point in history natural nuclear fission of U-235 occurred in these sites.  After examining the contents of the deposits, it was determined that these uranium deposits went critical about 2 billion years ago. These sites have been the subjects of much research in the years since their discovery. The conditions found in the Franceville basin were very similar to those predicted by Kuroda in 1956.
The natural nuclear fission reaction was determined to require at least 1% U-235, which has a half-life of 700 million years. Extrapolation leads one to determine that this composition could have occurred up until 400 million years ago.  However, there is reason that these sites have not been found throughout the world. There are other conditions needed to sustain the nuclear fission reaction. Four main conditions must be met in order for the spontaneous natural fission event to occur. 
The uranium deposit must be dense enough to reach critical mass
Surrounding compounds should not be "fission poisoning" (containing boron and other dampening rare earth elements [REEs])
Surrounding compounds must be plentiful in "light" nuclei that can moderate the nuclear reaction
The uranium must have a high concentration of fissionable elements; high ratio of (U-235 : U-238)
Uranium concentrations required for fission were found to be between 10 and 20%, additionally the space around the uranium deposits must have a porosity of between 10 and 15% in order for the necessary amount of water to be present to moderate the nuclear reaction. The geology of the Franceville Basin is the only known location to have possessed all of these criteria at one point. 
It was determined that these reactors sustained nuclear fission reactions for 105-106 years beginning about 2 billion years ago.  At this time the natural abundance of U-235 would have been ~3%.
On first inspection it appeared that there were two sites of natural nuclear fission in the Franceville basin. However, after more exploration and sample analysis, there are now 16 sites in the basin that have been found to possess the discrepant uranium ratios that are footprints of the fission reaction. Of these 16 sites (numbered 1-16) reactors 1 and 2 and 7-9 were found to have far different capacities for fission. Sites 1-2 were determined to have undergone fission of ~5×103 Kg uranium (producing 16500 MW/yr) while sites 7-9 were found to have undergone fission of 480 kg of uranium (producing 1300 MW/yr). The conditions surrounding reactors 1-2 were approximately 10 times more energetic than reactors 7-9 during the lifetime of the reactors. 
Due to the organic matter found in the sites at Gabon, it is theorized that these conditions allowed for encapsulation of the reactor relics. Upon criticality the temperature of the reactors has been determined to be around 350°C. Upon reaching this elevated temperature, the water in the area would evaporate and move away from the reactor, stopping the reaction. After the reaction stopped and the rectors cooled down, water would return and fission would start again. The water would carry organic material away from the reactor and when it cooled, it would solidify, essentially encasing the reactor. These reactors were preserved as closed systems (until about 1.2 billion years ago when major tectonic shifts occurred in the region). The abundance and composition of rare earth element products found surrounding the reactor corroborate the time of the reactors' criticality and the amount of U-235 that underwent fission. 
After further examination it appears that natural nuclear fission reactors are to be expected based on increased U-235 concentration throughout history. The four main conditions needed to sustain nuclear reaction for extended times are at present unattainable, based mainly on the low U-235 concentration from natural decay. The sites at Gabon have given us an insight into the natural nuclear fission reactors that existed about 2 billion years ago and also information about how the fission products have moved through the area in the many years since.
© Timothy Blake. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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