Uranium-bearing minerals are present in the underlying granite basement rock (Meshik, 2005). The Gabon Reactors were formed in a marine sandstone layer in the Franceville Basin. As the dissolved CO 2-content increases, so does the mobility of these uranium species. However, when uranium is in its oxidised form (U 6+), it easily forms soluble complexes. When uranium is in its reduced form (U 4+), uranium tends to form very stable compounds that are not easily brought into solution. For this hydrothermal circulation to concentrate uranium, that uranium must be soluble in order to be mobilised. Uranium is generally concentrated by hydrothermal circulation, which picks up uranium and concentrates it as a secondary hydrothermal deposit. In most rocks on earth, uranium is present only in trace quantities (ppm or ppb) in a number of minerals. At that time, the levels of oxygen in the atmosphere rose significantly, from <1% to ≥15%. The reason uranium only became concentrated enough around two billion years ago to initiate natural fission, has been linked to the ‘Great Oxidation Event’ that started around 2.4 billion years ago (Gauthier-Lafaye and Weber, 2003). In theory, an ancient uranium deposit could have spontaneously developed a self-sustaining nuclear fission, assuming the uranium was concentrated enough, there was a substance (most likely water) to act as a moderator, and there were not significant amounts of neutron-absorbing elements nearby. About two billion years ago, there would have been about 3.6% uranium-235 present in uranium ore on the earth’s crust, about the proportion of uranium-235 used in pressurised boiling water reactor nuclear power plants. It was suggested by Kuroda (1956) that the conditions necessary for a natural nuclear reactor to develop could have been present in ancient uranium deposits. The average power output of the Gabon reactors is suggested to have been equivalent to about 100kW, which would enough to power about 1 000 lightbulbs. The energy produced by these natural nuclear reactors was modest. It is suggested that the Gabon nuclear reactors spontaneously began operating around two billion years ago, and they continued to operate in a stable manner for up to one million years and the radioactive products of the nuclear fission have been safely contained over the entire period. Image credit: US Department of Energy Natural Fission Oklo natural reactor as seen in underground mining operations. In 1997 Francois Gauthier-Lafaye wrote a plea to the journal Nature, advocating that mining of the Bangombé uranium be stopped. In the late 1990s, there was danger that the last natural nuclear reactor at Bangombé would be mined as well. Only a limited number of specimens remained that were made available for study. An additional seventeenth natural nuclear reactor was also discovered at Bangombé, located about 30km to the south-east of Oklo.īy the time the significance of the discovery was realised by the scientific community, the sixteen natural nuclear reactors at Oklo had been destroyed, completely mined out for their rich uranium ore. Further exploration discovered sixteen natural nuclear reactors in uranium mines at Oklo. It was considered very important to the officials to account for this ‘missing’ uranium-235. However, there were high concentrations of elements like cesium, curium, americium and even plutonium to be found. The uranium ore was anomalously depleted in uranium-235, containing only 0.717%. During a routine isotopic measurement of uranium ore from Gabon, it was noticed that the uranium ore did not have a uranium-235 content of 0.720% as most other known deposits. The French had been mining uranium in Gabon, at Oklo, for several years to utilise in their nuclear power plants. The probability of natural fission was theorised by Paul Kuroda in a 1956 paper, but not proven until seventeen natural fission sites were discovered in Gabon in 1972 and became commonly known as the ‘Gabon Reactors’. The first man-made nuclear fission plants were only developed in the 1950s.
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