A natural nuclear fission reactor is a
uranium
Uranium is a chemical element; it has chemical symbol, symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Ura ...
deposit where self-sustaining
nuclear chain reaction
In nuclear physics, a nuclear chain reaction occurs when one single nuclear reaction causes an average of one or more subsequent nuclear reactions, thus leading to the possibility of a self-propagating series or "positive feedback loop" of thes ...
s occur. The idea of a
nuclear reactor
A nuclear reactor is a device used to initiate and control a Nuclear fission, fission nuclear chain reaction. They are used for Nuclear power, commercial electricity, nuclear marine propulsion, marine propulsion, Weapons-grade plutonium, weapons ...
existing ''in situ'' within an ore body moderated by groundwater was briefly explored by
Paul Kuroda in 1956. The existence of an extinct or fossil nuclear fission reactor, where self-sustaining nuclear reactions have occurred in the past, are established by analysis of
isotope
Isotopes are distinct nuclear species (or ''nuclides'') of the same chemical element. They have the same atomic number (number of protons in their Atomic nucleus, nuclei) and position in the periodic table (and hence belong to the same chemica ...
ratio
In mathematics, a ratio () shows how many times one number contains another. For example, if there are eight oranges and six lemons in a bowl of fruit, then the ratio of oranges to lemons is eight to six (that is, 8:6, which is equivalent to the ...
s of uranium and of the
fission product
Nuclear fission products are the atomic fragments left after a large atomic nucleus undergoes nuclear fission. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons, the releas ...
s (and the stable
daughter nuclides of those fission products). The first discovery of such a reactor happened in 1972 in
Oklo
Oklo is a region near Franceville in the Haut-Ogooué Province of Gabon. Several natural nuclear fission reactors were discovered in the uranium mines in the region in 1972.
History
Gabon was a French colony when prospectors from the Comm ...
,
Gabon
Gabon ( ; ), officially the Gabonese Republic (), is a country on the Atlantic coast of Central Africa, on the equator, bordered by Equatorial Guinea to the northwest, Cameroon to the north, the Republic of the Congo to the east and south, and ...
, by researchers from the
French Alternative Energies and Atomic Energy Commission
The French Alternative Energies and Atomic Energy Commission, or CEA ( French: Commissariat à l'énergie atomique et aux énergies alternatives), is a French public government-funded research organisation in the areas of energy, defense and sec ...
(CEA) when chemists performing quality control for the French nuclear industry noticed sharp depletions of
fissile
In nuclear engineering, fissile material is material that can undergo nuclear fission when struck by a neutron of low energy. A self-sustaining thermal Nuclear chain reaction#Fission chain reaction, chain reaction can only be achieved with fissil ...
in gaseous uranium made from Gabonese ore.
Oklo is the only location where this phenomenon is known to have occurred, and consists of 16 sites with patches of centimeter-sized
ore layers. There, self-sustaining
nuclear fission
Nuclear fission is a reaction in which the nucleus of an atom splits into two or more smaller nuclei. The fission process often produces gamma photons, and releases a very large amount of energy even by the energetic standards of radioactiv ...
reactions are thought to have taken place approximately 1.7
billion
Billion is a word for a large number, and it has two distinct definitions:
* 1,000,000,000, i.e. one thousand million, or (ten to the ninth power), as defined on the short scale. This is now the most common sense of the word in all varieties of ...
years ago, during the
Statherian
The Statherian Period (; , meaning "stable, firm") is the final geologic period in the Paleoproterozoic Era and lasted from Mya to Mya (million years ago). Instead of being based on stratigraphy, these dates are defined chronometrically.
Th ...
period of the
Paleoproterozoic
The Paleoproterozoic Era (also spelled Palaeoproterozoic) is the first of the three sub-divisions ( eras) of the Proterozoic eon, and also the longest era of the Earth's geological history, spanning from (2.5–1.6 Ga). It is further sub ...
. Fission in the ore at Oklo continued off and on for a few hundred thousand years and probably never exceeded 100
kW of thermal power.
Life on Earth at this time consisted largely of sea-bound
algae
Algae ( , ; : alga ) is an informal term for any organisms of a large and diverse group of photosynthesis, photosynthetic organisms that are not plants, and includes species from multiple distinct clades. Such organisms range from unicellular ...
and the first
eukaryotes
The eukaryotes ( ) constitute the domain of Eukaryota or Eukarya, organisms whose cells have a membrane-bound nucleus. All animals, plants, fungi, seaweeds, and many unicellular organisms are eukaryotes. They constitute a major group of ...
, living under a 2%
oxygen
Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...
atmosphere. However even this meager oxygen was likely essential to the concentration of uranium into fissionable ore bodies, as uranium dissolves in water only in the presence of oxygen. Before the planetary-scale production of oxygen by the early
photosynthesizers groundwater-moderated natural nuclear reactors are not thought to have been possible.
Discovery of the Oklo fossil reactors
In May 1972, at the
Tricastin uranium enrichment site at Pierrelatte, France, routine
mass spectrometry
Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a ''mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is used ...
comparing
UF6 samples from the Oklo mine showed a discrepancy in the amount of the isotope. Where the usual concentrations of were 0.72% the Oklo samples showed only 0.60%. This was a significant difference—the samples bore 17% less than expected.
This discrepancy required explanation, as all civilian uranium handling facilities must meticulously account for all fissionable isotopes to ensure that none are diverted into the construction of unsanctioned
nuclear weapon
A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission (fission or atomic bomb) or a combination of fission and fusion reactions (thermonuclear weapon), producing a nuclear exp ...
s. Further, as
fissile material
In nuclear engineering, fissile material is material that can undergo nuclear fission when struck by a neutron of low energy. A self-sustaining thermal chain reaction can only be achieved with fissile material. The predominant neutron energy i ...
is the reason for mining uranium in the first place, the missing 17% was also of direct economic concern.

Thus, the
French Alternative Energies and Atomic Energy Commission
The French Alternative Energies and Atomic Energy Commission, or CEA ( French: Commissariat à l'énergie atomique et aux énergies alternatives), is a French public government-funded research organisation in the areas of energy, defense and sec ...
(CEA) began an investigation. A series of measurements of the relative abundances of the two most significant
isotopes of uranium
Uranium (U) is a naturally occurring radioactive element (radioelement) with no stable isotopes. It has two primordial isotopes, uranium-238 and uranium-235, that have long half-lives and are found in appreciable quantity in Earth's crust. The d ...
mined at Oklo showed anomalous results compared to those obtained for uranium from other mines. Further investigations into this uranium deposit discovered uranium ore with a concentration as low as 0.44% (almost 40% below the normal value). Subsequent examination of isotopes of fission products such as
neodymium
Neodymium is a chemical element; it has Symbol (chemistry), symbol Nd and atomic number 60. It is the fourth member of the lanthanide series and is considered to be one of the rare-earth element, rare-earth metals. It is a hard (physics), hard, sli ...
and
ruthenium
Ruthenium is a chemical element; it has symbol Ru and atomic number 44. It is a rare transition metal belonging to the platinum group of the periodic table. Like the other metals of the platinum group, ruthenium is unreactive to most chem ...
also showed anomalies, as described in more detail below. However, the
trace radioisotope
A trace radioisotope is a radioisotope that occurs naturally in trace amounts (i.e. extremely small). Generally speaking, trace radioisotopes have half-lives that are short in comparison with the age of the Earth, since primordial nuclides tend to ...
did not deviate significantly in its concentration from other natural samples. Both
depleted uranium
Depleted uranium (DU), also referred to in the past as Q-metal, depletalloy, or D-38, is uranium with a lower content of the fissile isotope Uranium-235, 235U than natural uranium. The less radioactive and non-fissile Uranium-238, 238U is the m ...
and
reprocessed uranium
Reprocessed uranium (RepU) is the uranium recovered from nuclear reprocessing, as done commercially in France, the UK and Japan and by nuclear weapons states' military plutonium production programs. This uranium makes up the bulk of the material ...
will usually have concentrations significantly different from the
secular equilibrium
In nuclear physics, secular equilibrium is a situation in which the quantity of a radioactive isotope remains constant because its production rate (e.g., due to decay of a parent isotope) is equal to its decay rate.
In radioactive decay
Secular e ...
of 55
ppm relative to . This is due to being enriched together with and due to it being both consumed by
neutron capture
Neutron capture is a nuclear reaction in which an atomic nucleus and one or more neutrons collide and merge to form a heavier nucleus. Since neutrons have no electric charge, they can enter a nucleus more easily than positively charged protons, wh ...
and produced from by
fast neutron
The neutron detection temperature, also called the neutron energy, indicates a free neutron's kinetic energy, usually given in electron volts. The term ''temperature'' is used, since hot, thermal and cold neutrons are moderated in a medium with ...
induced (n,2n) reactions in nuclear reactors. In Oklo, any possible deviation of concentration present at the time the reactor was active would have long since decayed away. must have also been present in higher than usual ratios during the time the reactor was operating, but due to its half life of years being almost two orders of magnitude shorter than the time elapsed since the reactor operated, it has decayed to roughly its original value and below any abilities of current equipment to detect.
This loss in is exactly what happens in a nuclear reactor. A possible explanation was that the uranium ore had operated as a natural fission reactor in the distant geological past. Other observations led to the same conclusion, and on 25 September 1972, the CEA announced their finding that self-sustaining nuclear chain reactions had occurred on Earth about 2 billion years ago. Later, other natural nuclear fission reactors were discovered in the region.
Fission product isotope signatures
Neodymium
The neodymium found at Oklo has a different isotopic composition to that of natural neodymium: the latter contains 27% , while that of Oklo contains less than 6%. The is not produced by fission; the ore contains both fission-produced and natural neodymium. From this content, we can subtract the natural neodymium and gain access to the isotopic composition of neodymium produced by the fission of . The two isotopes and lead to the formation of and by neutron capture. This excess must be corrected (see above) to obtain agreement between this corrected isotopic composition and that deduced from fission yields.
Ruthenium
Similar investigations into the isotopic ratios of
ruthenium
Ruthenium is a chemical element; it has symbol Ru and atomic number 44. It is a rare transition metal belonging to the platinum group of the periodic table. Like the other metals of the platinum group, ruthenium is unreactive to most chem ...
at Oklo found a much higher concentration than otherwise naturally occurring (27–30% vs. 12.7%). This anomaly could be explained by the decay of to . In the bar chart, the normal natural isotope signature of ruthenium is compared with that for
fission product
Nuclear fission products are the atomic fragments left after a large atomic nucleus undergoes nuclear fission. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons, the releas ...
ruthenium which is the result of the
fission of with thermal neutrons. The fission ruthenium has a different isotope signature. The level of in the fission product mixture is low because fission produces
neutron rich isotopes which subsequently
beta decay
In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which an atomic nucleus emits a beta particle (fast energetic electron or positron), transforming into an isobar of that nuclide. For example, beta decay of a neutron ...
and would only be produced in appreciable quantities by
double beta decay
In nuclear physics, double beta decay is a type of radioactive decay in which two neutrons are simultaneously transformed into two protons, or vice versa, inside an atomic nucleus. As in single beta decay, this process allows the atom to move cl ...
of the very long-lived (half life years)
molybdenum
Molybdenum is a chemical element; it has Symbol (chemistry), symbol Mo (from Neo-Latin ''molybdaenum'') and atomic number 42. The name derived from Ancient Greek ', meaning lead, since its ores were confused with lead ores. Molybdenum minerals hav ...
isotope . On the timescale of when the reactors were in operation, very little (about 0.17
ppb) decay to will have occurred. Other pathways of production like
neutron capture
Neutron capture is a nuclear reaction in which an atomic nucleus and one or more neutrons collide and merge to form a heavier nucleus. Since neutrons have no electric charge, they can enter a nucleus more easily than positively charged protons, wh ...
in or (quickly followed by beta decay) can only have occurred during high
neutron flux
The neutron flux is a scalar quantity used in nuclear physics and nuclear reactor physics. It is the total distance travelled by all free neutrons per unit time and volume. Equivalently, it can be defined as the number of neutrons travelling ...
and thus ceased when the fission chain reaction stopped.
Mechanism
The natural nuclear reactor at Oklo formed when a uranium-rich mineral deposit became inundated with
groundwater
Groundwater is the water present beneath Earth's surface in rock and Pore space in soil, soil pore spaces and in the fractures of stratum, rock formations. About 30 percent of all readily available fresh water in the world is groundwater. A unit ...
, which could act as a
moderator for the neutrons produced by nuclear fission. A
chain reaction
A chain reaction is a sequence of reactions where a reactive product or by-product causes additional reactions to take place. In a chain reaction, positive feedback leads to a self-amplifying chain of events.
Chain reactions are one way that sys ...
took place, producing heat that caused the groundwater to boil away; without a moderator that could slow the neutrons, however, the reaction slowed or stopped. The reactor thus had a negative
void coefficient
In nuclear engineering, the void coefficient (more properly called void coefficient of reactivity) is a number that can be used to estimate how much the reactivity of a nuclear reactor changes as voids (typically steam bubbles) form in the reactor ...
of reactivity, something employed as a safety mechanism in human-made
light water reactor
The light-water reactor (LWR) is a type of thermal-neutron reactor that uses normal water, as opposed to heavy water, as both its coolant and neutron moderator; furthermore a solid form of fissile elements is used as fuel. Thermal-neutron react ...
s. After cooling of the mineral deposit, the water returned, and the reaction restarted, completing a full cycle every 3 hours. The fission reaction cycles continued for hundreds of thousands of years and ended when the ever-decreasing fissile materials, coupled with the build-up of
neutron poison
In applications such as nuclear reactors, a neutron poison (also called a neutron absorber or a nuclear poison) is a substance with a large neutron absorption cross-section. In such applications, absorbing neutrons is normally an undesirable ef ...
s, no longer could sustain a chain reaction.
Fission of uranium normally produces five known isotopes of the fission-product gas
xenon
Xenon is a chemical element; it has symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the ...
; all five have been found trapped in the remnants of the natural reactor, in varying concentrations. The concentrations of xenon isotopes, found trapped in mineral formations 2 billion years later, make it possible to calculate the specific time intervals of reactor operation: approximately 30 minutes of criticality followed by 2 hours and 30 minutes of cooling down (exponentially decreasing residual
decay heat
Decay heat is the heat released as a result of radioactive decay. This heat is produced as an effect of radiation on materials: the energy of the alpha particle, alpha, Beta particle, beta or gamma radiation is converted into the thermal movement ...
) to complete a 3-hour cycle.
Xenon-135
Xenon-135 (135Xe) is an Isotope#Radioactive, primordial, and stable isotopes, unstable isotope of xenon with a half-life of about 9.2 hours. 135Xe is a fission product of uranium and it is the most powerful known neutron-absorbing nuclear poison ...
is the strongest known neutron poison. However, it is not produced directly in appreciable amounts but rather as a decay product of
iodine-135
There are 40 known isotopes of iodine (53I) from 108I to 147I; all undergo radioactive decay except 127I, which is stable. Iodine is thus a monoisotopic element.
Its longest-lived radioactive isotope, 129I, has a half-life of 16.14 million yea ...
(or one of its
parent nuclides). Xenon-135 itself is unstable and decays to
caesium-135
Caesium (55Cs) has 41 known isotopes, ranging in mass number from 112 to 152. Only one isotope, 133Cs, is stable. The longest-lived radioisotopes are 135Cs with a half-life of 1.33 million years, with a half-life of 30.1671 years and 134Cs with a ...
if not allowed to absorb neutrons. While caesium-135 is relatively long lived, all caesium-135 produced by the Oklo reactor has since decayed further to stable
barium-135. Meanwhile, xenon-136, the product of
neutron capture
Neutron capture is a nuclear reaction in which an atomic nucleus and one or more neutrons collide and merge to form a heavier nucleus. Since neutrons have no electric charge, they can enter a nucleus more easily than positively charged protons, wh ...
in xenon-135 decays extremely slowly via
double beta decay
In nuclear physics, double beta decay is a type of radioactive decay in which two neutrons are simultaneously transformed into two protons, or vice versa, inside an atomic nucleus. As in single beta decay, this process allows the atom to move cl ...
and thus scientists were able to determine the neutronics of this reactor by calculations based on those isotope ratios almost two billion years after it stopped fissioning uranium.

A key factor that made the reaction possible was that, at the time the reactor went
critical
Critical or Critically may refer to:
*Critical, or critical but stable, medical states
**Critical, or intensive care medicine
* Critical juncture, a discontinuous change studied in the social sciences.
*Critical Software, a company specializing i ...
1.7 billion years ago, the
fissile
In nuclear engineering, fissile material is material that can undergo nuclear fission when struck by a neutron of low energy. A self-sustaining thermal Nuclear chain reaction#Fission chain reaction, chain reaction can only be achieved with fissil ...
isotope made up about 3.1% of the natural uranium, which is comparable to the amount used in some of today's reactors. (The remaining 96.9% was and roughly 55 ppm , neither of which is fissile by
slow or moderated neutrons.) Because has a shorter
half-life Half-life is a mathematical and scientific description of exponential or gradual decay.
Half-life, half life or halflife may also refer to:
Film
* Half-Life (film), ''Half-Life'' (film), a 2008 independent film by Jennifer Phang
* ''Half Life: ...
than , and thus decays more rapidly, the current abundance of in natural uranium is only 0.72%. A natural nuclear reactor is therefore no longer possible on Earth without
heavy water
Heavy water (deuterium oxide, , ) is a form of water (molecule), water in which hydrogen atoms are all deuterium ( or D, also known as ''heavy hydrogen'') rather than the common hydrogen-1 isotope (, also called ''protium'') that makes up most o ...
or
graphite
Graphite () is a Crystallinity, crystalline allotrope (form) of the element carbon. It consists of many stacked Layered materials, layers of graphene, typically in excess of hundreds of layers. Graphite occurs naturally and is the most stable ...
.
The Oklo uranium ore deposits are the only known sites in which natural nuclear reactors existed. Other rich uranium ore bodies would also have had sufficient uranium to support nuclear reactions at that time, but the combination of uranium, water, and physical conditions needed to support the chain reaction was unique, as far as is currently known, to the Oklo ore bodies. It is also possible that other natural nuclear fission reactors were once operating but have since been geologically disturbed so much as to be unrecognizable, possibly even "diluting" the uranium so far that the isotope ratio would no longer serve as a "fingerprint". Only a small part of the continental crust and no part of the oceanic crust reaches the age of the deposits at Oklo or an age during which isotope ratios of natural uranium would have allowed a self sustaining chain reaction with water as a moderator.
Another factor which probably contributed to the start of the Oklo natural nuclear reactor at 2 billion years, rather than earlier, was the
increasing oxygen content in the Earth's atmosphere.
Uranium is naturally present in the rocks of the earth, and the abundance of fissile was at least 3% or higher at all times prior to reactor startup. Uranium is soluble in water only in the presence of
oxygen
Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...
. Therefore, increasing oxygen levels during the aging of the Earth may have allowed uranium to be dissolved and transported with groundwater to places where a high enough concentration could accumulate to form rich uranium ore bodies. Without the new aerobic environment available on Earth at the time, these concentrations probably could not have taken place.
It is estimated that nuclear reactions in the uranium in centimeter- to meter-sized veins consumed about five tons of and elevated temperatures to a few hundred degrees Celsius.
Most of the non-volatile fission products and actinides have only moved centimeters in the veins during the last 2 billion years.
Studies have suggested this as a useful natural analogue for nuclear waste disposal. The overall
mass defect from the fission of five tons of is about . Over its lifetime the reactor produced roughly in thermal energy, including
neutrino
A neutrino ( ; denoted by the Greek letter ) is an elementary particle that interacts via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass is so small ('' -ino'') that i ...
s. If one ignores fission of plutonium (which makes up roughly a third of fission events over the course of normal burnup in modern human-made
light water reactor
The light-water reactor (LWR) is a type of thermal-neutron reactor that uses normal water, as opposed to heavy water, as both its coolant and neutron moderator; furthermore a solid form of fissile elements is used as fuel. Thermal-neutron react ...
s), then
fission product yield
Nuclear fission splits a heavy nucleus such as uranium or plutonium into two lighter nuclei, which are called fission products. Yield refers to the fraction of a fission product produced per fission.
Yield can be broken down by:
# Individual i ...
s amount to roughly of technetium-99 (since decayed to ruthenium-99), of
zirconium-93
Naturally occurring zirconium (40Zr) is composed of four stable isotopes (of which one may in the future be found radioactive), and one very long-lived radioisotope (96Zr), a primordial nuclide that decays via double beta decay with an observed h ...
(since decayed to
niobium
Niobium is a chemical element; it has chemical symbol, symbol Nb (formerly columbium, Cb) and atomic number 41. It is a light grey, crystalline, and Ductility, ductile transition metal. Pure niobium has a Mohs scale of mineral hardness, Mohs h ...
-93), of caesium-135 (since decayed to barium-135, but the real value is probably lower as its parent nuclide, xenon-135, is a strong neutron poison and will have absorbed neutrons before decaying to in some cases), of
palladium-107 (since decayed to silver), of
strontium-90
Strontium-90 () is a radioactive isotope of strontium produced by nuclear fission, with a half-life of 28.79 years. It undergoes β− decay into yttrium-90, with a decay energy of 0.546 MeV. Strontium-90 has applications in medicine a ...
(long since decayed to zirconium), and of
caesium-137
Caesium-137 (), cesium-137 (US), or radiocaesium, is a radioactive isotope of caesium that is formed as one of the more common fission products by the nuclear fission of uranium-235 and other fissionable isotopes in nuclear reactors and nucle ...
(long since decayed to barium).
Relation to the atomic fine-structure constant
The natural reactor of Oklo has been used to check if the atomic
fine-structure constant
In physics, the fine-structure constant, also known as the Sommerfeld constant, commonly denoted by (the Alpha, Greek letter ''alpha''), is a Dimensionless physical constant, fundamental physical constant that quantifies the strength of the el ...
''α'' might have changed over the past 2 billion years. That is because α influences the rate of various nuclear reactions. For example, captures a neutron to become , and since the rate of neutron capture depends on the value of ''α'', the ratio of the two
samarium
Samarium is a chemical element; it has symbol Sm and atomic number 62. It is a moderately hard silvery metal that slowly oxidizes in air. Being a typical member of the lanthanide series, samarium usually has the oxidation state +3. Compounds of s ...
isotopes in samples from Oklo can be used to calculate the value of ''α'' from 2 billion years ago.
Several studies have analysed the relative concentrations of radioactive isotopes left behind at Oklo, and most have concluded that nuclear reactions then were much the same as they are today, which implies that ''α'' was the same too.
See also
*
Deep geological repository
A deep geological repository is a way of storing hazardous or radioactive waste within a stable geologic environment, typically 200–1,000 m below the surface of the earth. It entails a combination of waste form, waste package, engineered seals ...
*
Geology of Gabon
*
Mounana
Mounana is a town in Gabon. It lies on the N3 road and from 1958 until the 1990s was a major uranium mining centre. The mine is now closed, and it is now primarily a centre for agriculture
Agriculture encompasses crop and livestock p ...
References
Sources
*
External links
The natural nuclear reactor at Oklo: A comparison with modern nuclear reactors, Radiation Information Network, April 2005Oklo Fossil Reactors*
הכור הגרעיני של הטבע(in
Hebrew language
Hebrew (; ''ʿÎbrit'') is a Northwest Semitic language within the Afroasiatic language family. A regional dialect of the Canaanite languages, it was natively spoken by the Israelites and remained in regular use as a first language unti ...
)
{{Nuclear fission reactors
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Geography of Gabon
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