• MINING IN FOCUS
GABON’S
NATURAL FISSION SITES
Nature has a way of doing things first, better and safer – even nuclear
fission, writes Nicolaas C Steenkamp.
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
Oklo natural reactor as seen in underground mining operations.
to the journal Nature, advocating that mining of the Bangombé
uranium be stopped.
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 (U 4+ ), uranium tends to
form very stable compounds that are not easily brought into
solution. However, when uranium is in its oxidised form (U 6+ ), it
easily forms soluble complexes. As the dissolved CO 2
-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
34 • African Mining •May 2020
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