African Mining May 2020 | Page 37

MINING IN FOCUS • the underlying granite basement rock (Meshik, 2005). The sandstone unit was infiltrated by oxidising water, which dissolved the uraniumbearing minerals along the bottom contact and mobilised and concentrated the uranium in several deposits towards the top of the sandstone layer. The uranium content, in fact, became extraordinarily well-concentrated. Fission of uranium could have begun when the uranium concentration reached 10%; the Gabon uranium deposits in which the natural nuclear reactors developed is estimated to have contained about 25% to 60% uranium (Meshik, 2005). The Gabon reactors were able to meet the requirements that Kuroda (1956) had suggested, in as far as that there were high enough concentrations of uranium which still contained a significant amount of highlyfissionable uranium-235 and water was able to percolate into the permeable sandstone containing the uranium deposits. This water acted as the neutron moderator, slowing neutrons down so that they were more likely to hit atomic nuclei and cause fission reactions. There appears to also have been no significant quantities of neutron-absorbing elements to inhibit the self-sustaining fission reaction (Meshik, 2005). It is suggested that the Gabon Reactors would have been active over a period of several hundred thousand years and would have acted like modern geysers. For approximately 30 minutes the reaction would go critical, with fission proceeding until the water boils away. Over the next ~150 minutes, there would be a cooldown period, after which water would flood the reactive zone again and fission would restart. This interpretation is based on examining the concentrations of xenon isotopes that become trapped in the mineral formations surrounding the uranium ore deposits. Eventually, the fissionable uranium-235 was depleted to such an extent that the Gabon natural reactors became inactive. The last factor that has significant scientific value to both geologist and nuclear scientist is the fact that the Gabon natural reactors, over their entire life, never contaminated large areas of country rock surrounding it. The natural nuclear reactors in Gabon seem to have been largely protected by enveloping carbonaceous substances and clay, which created and maintained reducing (low oxygen) conditions which largely inhibited the movement of uranium and other radioactive by-products of nuclear fission. In addition, it was found that the plutonium and cesium fission by-product was effectively captured. According to an article in Forbes, barium (the 'trace' element left after the full breakdown of the plutonium and Mossman et al., 2008 Oklo site simplified geological setting. cesium) is not found evenly distributed in the country rock, but rather found in nests surrounded by a thin layer of rutheniumcompounds. It has been suggested that containers made of ruthenium alloys could be used to safely store radioactive waste for a very long time and would be resistant to exposure to radioactive material and corrosion by water over vast geological periods. The problem, however, is that ruthenium is expensive and rare. Research is underway to examine the molecular structure of the ruthenium that’s holding onto the radioactive cesium to better understand how the two elements are bound together. It is hoped that this research will provide a way to adapt iron to hold onto the radioactive elements in spent nuclear fuel. • References: 1. Forbes, https://www.forbes.com/sites/davidbressan/2018/08/14/ two-billion-year-old-natural-reactor-may-holds-key-for-safe-nuclearwaste-disposal/#24a604a93c72 2. Gauthier-Lafaye F. (1997). The last natural nuclear fission reactor. Nature, vol. 387: 337. 3. Gauthier-Lafaye, F. and Weber, F. (2003). Natural nuclear fission reactors: Time constraints for occurrence and their relation to uranium and manganese deposits and to the evolution of the atmosphere. 4. Precambrian Research, vol. 120, no. 1-2: 81-101. 5. Kuroda, P. (1956). On the nuclear physical stability of uranium minerals. Journal of Chemical Physics, vol. 25: 781-782. 6. Meshik, A. 2005. The Workings of an Ancient Nuclear Reactor. Scientific American, vol. 293, no. 5: 82-91 7. Mossman D.J.; Gauthier-Lafaye, F.; Dutkiewicz A. and Brüning, R. (2008). Carbonaceous substances in Oklo reactors—Analogue for permanent deep geologic disposal of anthropogenic nuclear waste. 8. Reviews in Engineering Geology, vol. 19: 1-13. For the latest industry and association news, events/exhibitions information, articles, etc. visit our website WWW.AFRICANMINING.CO.ZA Read it all online. www. africanmining.co.za African Mining Publication African Mining African Mining • May 2020 • 35