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