ENS President's contributions
The nuclear Reactors of Oklo: 2 billion Years before
Bertrand Barré, President European Nuclear
1. "The Italian Navigator has landed in the
On December 2, 1942, this cryptic message announced
that the team gathered around Enrico Fermi in Chicago had managed
to sustain a fission chain reaction in the first ever man made
nuclear reactor, CP1. This was the climax of a decade long search,
starting with the discovery by Chadwick in 1932 of the neutron,
a particle able to interact with the nuclei without being hampered
by their electric charges, the series of experiments by Fermi
sending “moderated” neutrons against every nucleus
of the Mendeleyev Table, the discovery of the fission of uranium
by Otto Hahn, Lise Meitner and Fritz Straßmann in 1938.
When the team led by Joliot discovered, a few months later, that
2 to 3 new neutrons were emitted during the fission, they were
able to conceptually design a nuclear reactor, a facility using
a sustained fission chain reaction to generate vast amounts of
energy, but World War 2 shifted the research efforts to America.
And for three decades, it was believed that CP1
was not only the first man made reactor, but the first nuclear
reactor ever – full stop.
2. Radioactive Earth.
Not everybody realizes that geothermal energy
is just another name to describe the radioactivity of our planet.
Among the heavier elements retained during the formation of Earth
(most of the lightest elements escaped its too small gravity),
a number have only radioactive isotopes. Potassium
1 , Thorium and Uranium are the most abundant remaining
today. The energy they keep releasing during their radioactive
decay is the central heating system which supplements what we
receive from the Sun.
Natural uranium is (today) composed of three
major isotopes, 238U (abundance 99.2744%), 235U
(abundance 0.7202%) and 234U (abundance 0.0054%). This
very precise composition is the same – almost – everywhere
on Earth. All these isotopes are radioactive and decay with time,
but not with the same speed. The half-life of 238U
is 4.51 billion years while 235U decays by half in
“only” 710 million years. Therefore, the relative
abundance of 235U increases if we go back in time:
at the creation of the solar system, it was close to 17%, and
about 3.58% two billion years ago. 3.5% is the level to which
we painfully enrich the uranium today to fuel our Light Water
Reactors… In the 50s, some authors played with the idea
that fission chain reactions could have occurred naturally when
the enrichment was so high, but so many conditions would have
been required that it seemed far fetched, and there was no evidence
3. A Nuclear Detective Story
In June 1972, at the Pierrelatte enrichment plant
devoted to Defense Applications, a routine mass spectrometry analysis
of UF6 feed material exhibited a discrepancy: only 0.7171% of
the uranium in the samples 235U, instead of the magic
0.7202 ! Even though the discrepancy was small, it was so unusual
that the French Atomic Energy commission CEA, operator of the
plant, started a thorough investigation. First, it was not an
artifact: the anomaly was confirmed on several measurements on
other samples. Accidental contamination by depleted uranium from
the plant itself was then eliminated and so was the use of reprocessed
uranium as there was no 236U in the samples. The investigators
then traced the anomaly back through all the stages of uranium
processing, from Pierrelatte to Malvesi to Gueugnon where the
concentrates exhibited the same low 235U concentrations.
These concentrates all came from COMUF which operated two uranium
mines in Gabon, at Mounana and Oklo, the mill being located at
Mounana. Very soon it appeared that all the anomalous ore came
from the northern part of the – very rich – Oklo deposit.
In some shipments, the level of 235U was as low as
0.44%. Between 1970 and 1972, in the 700 tons of uranium delivered
by the Mounana mill, the deficit of 235U exceeded 200
kg, hardly a trifle !
Oklo mine uranium was indeed different from
natural uranium everywhere else. Why ?
“Natural” isotopic separation was
excluded : if it had produced depleted uranium, where was the
enriched fraction ? As soon as August, the hypothesis of very
ancient fission chain reactions was formulated, and investigators
started to search for fission products (or, rather, the granddaughters
of hypothetical fission products. The spectrum of fission products
is so distinctive that it constitutes an unmistakable marker that
fission reactions have taken place. The presence of such fission
products was clearly identified : at some point in the uranium
deposit history, it had become a “natural” nuclear
reactor. The discovery was duly heralded but many questions remained.
When did the reactor “start”? How long did it “operate”
? How was it “controlled”? The detective story was
Later on, it was found that there were actually
15 reactor sites in Oklo, and another one in Bangombé,
30 kilometers away from the main deposit.
4. Current answers to some questions about Oklo.
To run a nuclear reactor, you need a high concentration
of uranium with a minimum percentage of 235U
2, you need water to slow down the neutrons
3 and evacuate the calories and you must avoid
those elements which absorb neutrons greedily like boron, cadmium,
hafnium, gadolinium and other “poisons”. You need
also a minimum size (in the case of a deposit, a minimum thickness
of the seam) to prevent too many neutrons from escaping from the
It is only around 2.2 billion years ago that
the patient work of photosynthesis accomplished by the first algae
released enough oxygen in our atmosphere for the surface waters
and ground water to become oxidizing. Only then could the uranium
diluted in granite be leached out and concentrated before mineralization
in places where oxido-reduction would occur. Rich deposits cannot
be older. On the other hand, since 1.5 billion years, 235U
abundance has decayed below a level which makes spontaneous fission
workable. It took a lot of studies, in geology, chemistry and
reactor physics to narrow the bracket of time to the present estimated
value : the reactions must have started 1 950 ±
30 million years ago.
The deposits were located in very porous sandstone
where the ground water concentration may have been as high as
40%, probably due to the partial leaching of the silica (quartz
particles) by the hot groundwater, at a time where, the radioactivity
of Earth being higher than today, the thermal gradient underground
was probably higher too. During the reactors operation, the water
temperature rose significantly, accelerating this “de-silicication”
process and, by difference, increasing the concentration in uranium,
therefore compensating for its depletion by fission. As a matter
of fact, the concentration of uranium in the reaction zones is
extremely high, sometimes above 50%, and the higher the uranium
concentration, the lower its 235U content. Furthermore,
losing its silica, the surrounding sandstone became clay and thus
prevented an excessive migration of groundwater and keeping the
uranium in place.
From the fine analysis of the spectrum
of fission products, we know that a number of the fissions
occurred in plutonium, bred by neutron capture in 238U
and now fully decayed to 235U since its half-life
is only 24 000 years (By the way, so much for the notion
that plutonium is “artificial”). This allowed
the physicists to calculate that, varying from one zone
to another, reactions did take place during an enormous
period of time ranging from 150 000 to 850 000 years
The reactors where “controlled” by
several mechanisms, the main one being temperature : as the fission
power was released, the temperature rose. Higher temperature means
both an increase in absorption of neutrons (without fission) by
238U and a decrease in the efficiency of water as a
moderator : at a given temperature level, a level varying with
time and the progressive depletion of fissile uranium, the reactions
stabilize, as they do in our reactors 4.
By combining geology and temperature considerations,
it is now believed that the reactors in the northern part of the
deposit operated at a depth of several thousand meters,
under deltaic then marine sediments. At such depth, the conditions
of pressure and temperature were close to those of the Pressurized
Water Reactors of today (350 to 400°C, 15 to 25 Mpa), while
the southern zones operated at roughly 500 meters deep, with conditions
resembling more to those of a Boiling Water Reactor (250°C,
5 Mpa) 5: even the Oklo designers
did not choose between the present fierce competitors !
Even though significant alteration occurred in
recent times when the tectonic uprising and erosion brought the
reactors close to the surface, and especially when the Okolo Néné
River gouged the valley, the heavy elements thorium, uranium and
plutonium did not move at all, nor did the rare earths fission
products, as well as zirconium, ruthenium, palladium, rhodium
and a few others. On the other hand, krypton, xenon, iodine, barium
and strontium have moved, but maybe only after a few million years.
5. Oklo as a “natural analogue”
of a radioactive Waste Disposal Site ?
Soon after the discovery, and beyond the pure
scientific thrill, the nuclear community was very excited by its
implications, notably as a “natural analogue” for
the geologic disposal of High Level radioactive Waste (HLW).
There is more and more an international consensus
that the best way to dispose of HLW issued from the production
of electricity by nuclear reactors is to install them, with a
proper conditioning and packaging and additional engineered barriers,
in a stable underground geologic stratum where the radioactive
decay will progressively reduce their toxicity to a harmless level.
But this decay takes a long time, and it is quite a challenge
to demonstrate the containment of the radioactive products over
such a long period of time, ranging from tens to hundreds of thousands
of years. It can only be done through physico-mathematical modeling,
with the inherent uncertainties associated with the completeness
and accuracy of the models and their propagation along the calculations.
There, in Oklo, Mother Nature had contained precisely
the same radioactive elements not for hundreds of
thousands, not for millions, but for a couple of billion years,
and without engineered barriers or special packaging.
So much is true, especially for the heavier elements
which constitute most of the radiotoxicity of the HLW packages
6. But the comparison cannot be pushed
too far. To use a teenager’s expression, the Oklo reactors
are “too much”… If we could find a similar phenomenon
one million years old, that would be perfect, but we have seen
this is physically hopeless. For instance, most of the migration
occurred during the reactions themselves, over close to a million
years, when the conditions were far more troubled than what we
expect in a steady and cozy disposal facility : the site has been
deeply modified, losing by de-silicication three quarters of its
substance, minerals have been altered by irradiation, temperature
have run high and significant water convection did occur! Let
us say Oklo provides a good presumption, but not a demonstration.
6. Conclusion : A unique Phenomenon ?
Let me borrow my conclusion from the foreword
by the late Jules Horowitz to the book by Roger Naudet  which
I have used extensively for this paper : “It is after
all plausible that fission chain reactions might have spontaneously
occurred about two billion years ago, during a period of time
long enough to provoke locally significant anomalies in the isotopic
composition of some elements, notably uranium. What constitutes
a miracle is that, despite the upheavals that the Earth surface
has undergone since this ancient era, the evidence did survive
to our time, in Oklo, to be discovered owing to the watchfulness
of the CEA analysts”.
There is no reason to believe that what occurred
at lest 16 times near Oklo did not happen anywhere else on the
Earth, especially in old and rich deposits like exist in Australia
or Canada… but more than three decades after its discovery
Oklo remains unique. It remains unique as a geologic curiosity,
and it remains unique as a nuclear detective story.
40K in our bones
is responsible for half of the radioactivity of our own body,
which amounts to about 8000 Bq for an adult.
You can operate reactors with
natural uranium but only if you use heavy water D2O or very
pure graphite as moderator and a specific “heterogeneous”
fuel/moderator pattern, like in CANDU and Magnox types. It
would be very unlikely to find such pattern in nature.
Neutrons emitted during fission
move too fast to split easily other nuclei, but if the neutrons
can “bounce” off the nuclei of a moderator, this
will slow them down and make further fission more likely.
Radioactive decay of some
absorbing fission product also played a role over such long
If the operating time was
immense, the power density in the « core » was
only one millionth of its value in a commercial reactor today.
They have been retained within
the UO2 crystallites themselves
A few References
The Discovery (September 1972)
 R. Bodu et al. Sur l’existence d’anomalies
isotopiques rencontrées dans l’uranium du Gabon.
CR Académie des Sciences Paris 275 D p.1731
 M. Neuilly et al. Sur l’existence dans un passé
reculé d’une réaction en chaîne naturelle
de fissions dans le gisement d’uranium d’Oklo (Gabon)
 R. Naudet OKLO : Des réacteurs nucléaires
fossiles. Etude physique. Eyrolles, Paris, 1991
(with many interesting links !)
OKLO%20REACTORS.ppt (PowerPoint™ presentation –
I have used part of it)
(all about uranium)
(Oklo and HLW disposal)