Creating a nuclear reaction is not simple. In power plants, it involves
splitting uranium atoms, and that process releases energy as heat
and neutrons that go on to cause other atoms to split. This splitting
process is called nuclear fission. In a power plant, sustaining
the process of splitting atoms requires the involvement of many
scientists and technicians.
It came as a great surprise to most, therefore, when, in 1972, French
physicist Francis Perrin declared that nature had beaten humans
to the punch by creating the world’s first nuclear reactors.
Indeed, he argued, nature had a two-billion-year head start.1
Fifteen natural fission reactors have been found in three different
ore deposits at the Oklo mine in Gabon, West Africa. These are collectively
known as the Oklo Fossil Reactors.2
And when these deep underground natural nuclear chain reactions
were over, nature showed that it could effectively contain the radioactive
wastes created by the reactions.
No nuclear chain reactions will ever happen in a repository for
high-level nuclear wastes. But if a repository were to be built
at Yucca Mountain, scientists would count on the geology of the
area to contain radionuclides generated by these wastes with similar
In the early 1970s, French scientists noticed something odd about
samples of uranium recovered from the Oklo mine in Gabon, West Africa.
All atoms of a specific chemical element have the same chemical
properties, but may differ in weight; these different weights of
an element are known as isotopes. Some uranium samples from Gabon
had an abnormally low amount of the isotope U-235, which can sustain
a chain reaction. This isotope is rare in nature, but in some places,
the uranium found at Oklo contained only half the amount of the
isotope that should have been there.3
Scientists from other countries were skeptical when first hearing
of these natural nuclear reactors. Some argued that the missing
amounts of U-235 had been displaced over time, not split in nuclear
fission reactions. "How," they asked, "could fission
reactions happen in nature, when such a high degree of engineering,
physics, and acute, detailed attention went into building a nuclear
Perrin and the other French scientists concluded that the only other
uranium samples with similar levels of the isotopes found at Oklo
could be found in the used nuclear fuel produced by modern reactors.
They found that the percentages of many isotopes at Oklo strongly
resembled those in the spent fuel generated by nuclear power plants,
and, therefore, reasoned that a similar natural process had occurred.4
The uranium in the Earth contains dominantly two uranium isotopes,
U-238 and U-235, but also a very small percentage of U-234, and
perhaps small, undetectable amounts of others. All of these isotopes
undergo radioactive decay, but they do so at different rates. In
particular, U-235 decays about six-and-a-third times faster than
U-238. Thus, over time the proportion of U-235 to U-238 decreases.
But this change is slow because of the small rates of decay.
Generally, uranium isotope ratios are the same in all uranium ores
contained in nature, whether found in meteorites or in moon rocks.
Therefore, scientists believe that the original proportions of these
isotopes were the same throughout the solar system. At present,
U-238 comprises about 99.3 percent of the total, and U-235 comprises
about 0.7 percent.5 5 Any change in this ratio indicates
some process other than simple radioactive decay.
Calculating back to 1.7 billion years ago—the age of the deposits
in Gabon—scientists realized that the U-235 there comprised
about three percent of the total uranium. This is high enough to
permit nuclear fissions to occur, providing other conditions are
Deep under African soil, about 1.7 billion years ago, natural conditions
prompted underground nuclear reactions. Scientists from around the
world, including American scientists have studied the rocks at Oklo.
These scientists believe that water filtering down through crevices
in the rock played a key role. Without water, it would have been
nearly impossible for natural reactors to sustain chain reactions.
The water slowed the subatomic particles or neutrons that were cast
out from the uranium so that they could hit-and split-other atoms.
Without the water, the neutrons would move so fast that they would
just bounce off, like skipping a rock across the water, and not
produce nuclear chain reactions. When the heat from the reactions
became too great, the water turned to steam and stopped slowing
the neutrons. The reactions then slowed until the water cooled.
Then the process could begin again.7
Scientists think these natural reactors could have functioned intermittently
for a million years or more. Natural chain reactions stopped when
the uranium isotopes became too sparse to keep the reactions going.
Once the natural reactors burned themselves out, the highly radioactive
waste they generated was held in place deep under Oklo by the granite,
sandstone, and clays surrounding the reactors’ areas. Plutonium
has moved less than 10 feet from where it was formed almost two
billion years ago.8
Today, manmade reactors also create radioactive elements and by-products.
Scientists involved in the disposal of nuclear waste are very interested
in Oklo because long-lived wastes created there remain close to
their place of origin.
The Oklo phenomenon gives scientists an opportunity to examine the
results of a nearly natural two billion-year experiment, one that
cannot be duplicated in the lab. By analyzing the remnants of these
ancient nuclear reactors and understanding how underground rock
formations contained the waste, scientists studying Oklo can apply
their findings to containing nuclear waste today. The rock types
and other aspects of the geology at Oklo differ from those at Yucca
Mountain. But this information is useful in the design of a repository
at Yucca Mountain. Were the Oklo reactors a unique event in natural
history? Probably not. Scientists have found uranium ore deposits
in other geological formations of approximately the same age, not
only in Africa but also in other parts of the world, particularly
in Canada and northern Australia. But to date, no other natural
nuclear reactors have been identified.
Scientists believe that similar spontaneous nuclear reactions could
not happen today because too high a proportion of the U-235 has
decayed. But nearly two billion years ago, nature not only appears
to have created her first nuclear reactors, she also found a way
to successfully contain the waste they produced deep underground.
The radioactive remains of natural nuclear fission chain reactions
that happened 1.7 billion years ago in Gabon, West Africa, never
moved far beyond their place of origin. They remain contained in
the sedimentary rocks that kept them from being dissolved or spread
by groundwater. Scientists have studied Yucca Mountain to see if
the geology there might play a similar role in containing high-level
References 1 Cowan, G. A. 1976. "A Natural
Fission Reactor," Scientific American, 235:36.
2 Smellie, John. "The Fossil Nuclear Reactors of
Oklo, Gabon," Radwaste Magazine, Special Series on Natural
Analogs, March 1995:21.
3 "A Prehistoric Nuclear Reactor," Chemistry,
4 Smellie, 21.
5 Cowan, 41.
6 Smellie, 21.
7 Cowan, 39.
8 Cowan, 39.
Note: In 1956 while at the University of Arkansas, Dr. Paul Kuroda
described the conditions under which a natural nuclear reactor could
occur. When the Oklo reactors were discovered in 1972, the conditions
found there were very similar to his predictions. Dr. Kuroda now
lives in Las Vegas, Nevada where he has been a scientific resource
for the United States Department of Energy.