Gabon’s natural fission sites

2020-05-18T14:20:03+00:00 May 18th, 2020|Mining in Focus|

Nature has a way of doing things first, better and safer – even nuclear fission, writes Nicolaas C Steenkamp.

Oklo natural reactor as seen in underground mining operations. Image credit: US Department of Energy

Oklo natural reactor as seen in underground mining operations. Image credit: US Department of Energy

The first man-made nuclear fission plants were only developed in the 1950s. 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 French had been mining uranium in Gabon, at Oklo, for several years to utilise in their nuclear power plants. 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 uranium ore was anomalously depleted in uranium-235, containing only 0.717%. However, there were high concentrations of elements like cesium, curium, americium and even plutonium to be found. It was considered very important to the officials to account for this ‘missing’ uranium-235. Further exploration discovered sixteen natural nuclear reactors in uranium mines at Oklo. An additional seventeenth natural nuclear reactor was also discovered at Bangombé, located about 30km to the south-east of Oklo.

By 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. Only a limited number of specimens remained that were made available for study. In the late 1990s, there was danger that the last natural nuclear reactor at Bangombé would be mined as well. In 1997 Francois Gauthier-Lafaye wrote a plea to the journal Nature, advocating that mining of the Bangombé uranium be stopped.

Oklo natural reactor as seen in underground mining operations. Image credit: US Department of Energy 

Natural Fission

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. The energy produced by these natural nuclear reactors was modest. 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.

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. 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. 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.

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). At that time, the levels of oxygen in the atmosphere rose significantly, from <1% to ≥15%. In most rocks on earth, uranium is present only in trace quantities (ppm or ppb) in a number of minerals. Uranium is generally concentrated by hydrothermal circulation, which picks up uranium and concentrates it as a secondary hydrothermal deposit. For this hydrothermal circulation to concentrate uranium, that uranium must be soluble in order to be mobilised. When uranium is in its reduced form (U4+), uranium tends to form very stable compounds that are not easily brought into solution. However, when uranium is in its oxidised form (U6+), it easily forms soluble complexes. As the dissolved CO2-content increases, so does the mobility of these uranium species.

The Gabon Reactors were formed in a marine sandstone layer in the Franceville Basin. Uranium-bearing minerals are present in the underlying granite basement rock (Meshik, 2005). The sandstone unit was infiltrated by oxidising water, which dissolved the uranium-bearing 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).

Oklo site simplified geological setting. Image credit: Mossman et al., 2008

Oklo site simplified geological setting. Image credit: Mossman et al., 2008

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 highly-fissionable 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 cesium) is not found evenly distributed in the country rock, but rather found in nests surrounded by a thin layer of ruthenium-compounds. 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.