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
effectiveness.
Nature's reactors
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 reactor?"
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
Uranium isotopes decay at different
levels
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 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
right.6
So how did nuclear reactions occur in
nature?
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.
What happened to the nuclear waste left at
Oklo?
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
nuclear waste.
Source : US/DOE/2004
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References