The 2 Billion Year Old Natural Nuclear Fission Reactor of Gabon
The nuclear age might well have begun in America, but the world’s first fission reaction began in Gabon.
Gabon is one of the richest countries in Sub-Saharan Africa, with a per capita GDP four times that of its neighbours. Its economy is driven by oil, followed by wood and manganese exports.
Gabon has briefly exported uranium, a precious raw element needed in nuclear power facilities and nuclear weapons. The mines are no longer active, yet over two billion years ago, there was enough uranium in the rocks to trigger spontaneous nuclear fission.
Seventeen natural nuclear fission reactors functioned in what is now Gabon in Western Africa two billion years ago, aeons before humanity created the first commercial nuclear power plants in the 1950s.
The amount of electricity produced by these natural nuclear reactors was small. The Gabon reactors’ average power production was around 100 kilowatts, enough to power around 1,000 lightbulbs. In comparison, commercial pressurized boiling water reactor nuclear power facilities generate around 1,000 megawatts, enough to power approximately ten million light bulbs.
One of the most fascinating geological structures on the earth is the Oklo-reactor in Gabon, Africa. Natural fissile elements found in two billion-year-old rocks have sustained a slow nuclear fission process similar to that observed in a contemporary nuclear reactor.
The Formation of Oklo Nuclear Reactor
It was found in 1972 when a group of French scientists tested uranium ore from a mine in Gabon. Uranium ore is typically composed of three varieties (isotopes) of uranium, each with a different number of neutrons: uranium 238, the most abundant, uranium 234, the rarest, and uranium 235, which nuclear physicists are most interested in due to its ability to sustain nuclear chain reactions.
The uranium ore should include 0.720% uranium 235, which is the proportion found in various rock samples from the Earth’s crust, the Moon, and even meteorites. However, the French scientists discovered something unusual: the uranium sample contained just 0.717% uranium 235. In the case of uranium, what appears to be a little difference of 0.003% is quite important.
Uranium-235 has a half-life of 700 million years and is a radioactive element. It is present in practically all rocks, particularly magmatic rocks, and its disintegration is thought to be one of the sources of Earth’s interior heat. Because it decays at a steady pace throughout time, its concentration in the Earth’s crust is nearly constant everywhere – except in Oklo.
A massive river deposited the Oklo-Formation, a sequence of sandstone and siltstone, two billion years ago. The element uranium, obtained from worn magmatic rocks, became concentrated in specific layers of the sediments due to the microbial activity of the earliest lifeforms. Later tectonic processes buried the strata beneath the earth’s crust.
Chemical research revealed an exceptionally low proportion of uranium-235 in the ore mined at the Oklo open-pit mine in 1972. However, large amounts of elements such as cesium, curium, americium, and even plutonium were discovered. Today, such elements are only generated in nuclear reactors as uranium decays during controlled nuclear fission.
When uranium-235 decays, three neutrons are released. If one of the expelled neutrons collides with another uranium atom, that atom will also decay, starting a chain reaction. Most rocks either lack enough uranium to support nuclear fission or degrade too quickly to produce a chain reaction.
The Workings of a Natural Nuclear Reactor
Following this astounding finding, researchers continued examining the uranium mines in Oklo for more evidence, finally discovering at least sixteen locations in this region where spontaneous nuclear fission had occurred.
Researchers were able to piece together the intricacies of how these primitive reactors would have worked over the following several years.
Because of the biological activity of cyanobacteria, the oxygen concentration of the earth’s atmosphere grew a hundredfold around 2.4 billion years ago. This enabled the conversion of uranium from its insoluble state to its soluble oxide. Rainfall and natural sources of water dissolved the uranium and deposited it in sandstone layers, where it became concentrated enough to start a chain reaction. The water in the mine itself was important in keeping the reaction going.
Water delayed the released neutrons, allowing them to be absorbed by other nuclei and cause fission. The neutrons would have just bounced off the atoms without the water. When the heat from nuclear fission grew too intense, it boiled away all the water, and the process came to a halt. When the water returned, the procedure began again. These intervals of activity and inactivity were most likely brief. According to the calculations, the reactors were “on” for 30 minutes and then “off” for nearly 3 hours.
The Gabon reactors worked sporadically like this for a million years or more, until the uranium concentration became too low to sustain the reactions. Scientists believe that the average power output of the reactors was probably less than 100 kilowatts based on the quantity of uranium-235 utilized in the reactors.
Because of the African craton‘s remarkable stability, these old natural reactors have shifted relatively little from their original places. In one example, plutonium, one of the reaction’s byproducts, was discovered fewer than 10 feet from where it was produced two billion years ago.
Natural nuclear reactors may have existed in numerous different locations on Earth billions of years ago, but have since been eroded or absorbed by the earth’s crust. Gabon’s natural nuclear reactors are definitely one-of-a-kind.
Two variables combined in the Oklo-reactor to maintain slow nuclear fission for hundreds of millions of years. Weathering of magmatic rocks and bacterial activity concentrated the uranium to the point where it could initiate a nuclear chain reaction. Then, when water invaded the formation along faults, it slowed the radiated neutrons sufficiently to support slow and steady nuclear fission.
As uranium decays, it produces additional radioactive elements that power the reactor. The Oklo-reactor produced huge amounts of dangerous plutonium and cesium-isotopes over time, which have subsequently decomposed into stable and innocuous barium. During this procedure, however, no hazardous radioactivity had escaped into the environment.
A study published in the journal PNAS by a team of experts from the US Naval Research Laboratory in Washington, DC, looked into how the Oklo-reactor could run for so long without polluting the environment. In rocks recovered from the Oklo mine, barium (the ‘trace’ left by the prior radioactive elements) is found in nests surrounded by a thin coating of ruthenium compounds, rather than equally dispersed.
Ruthenium is a rare and inert metal that is frequently found in ore containing other elements. The scientists think that the radioactive plutonium and cesium were encapsulated and safely protected from the environment by a shell of ruthenium compounds. If this is the case, ruthenium alloy containers might be used to safely store radioactive waste for an extended period of time.
The Oklo-reactor indicates that ruthenium compounds remain stable even when exposed to radioactivity and corrosion by water over long geological periods.
What Happened to the Nuclear Waste left at Oklo?
When the natural reactors burnt out, the extremely radioactive waste they produced was trapped deep beneath Oklo by the sandstone, granite, and clays that surrounded the reactors’ regions. Plutonium has moved fewer than ten feet since its formation over two milliards years ago.
Man-made reactors now produce radioactive materials and by-products. Oklo is of particular interest to nuclear waste disposal scientists because the long-lived wastes generated there remain near their source.
The Oklo phenomenon allows scientists to investigate the outcomes of a virtually natural two billion-year experiment that cannot be replicated in the lab. Scientists researching Oklo can apply their results to preserving nuclear waste today by evaluating the relics of these ancient nuclear reactors and learning how subsurface geological formations kept the trash.
The geology of Oklo differs from that of Yucca Mountain in terms of rock kinds and other features. This knowledge, however, will be important in the design of a repository at Yucca Mountain. Were the Oklo reactors a one-of-a-kind natural event? Most likely not.
Scientists have discovered uranium ore concentrations in other geological formations of similar age, not just in Africa but also in other areas of the world, most notably in Canada and northern Australia. However, no further natural nuclear reactors have been discovered yet.
Scientists think that such spontaneous nuclear events could not occur now because too much U-235 has decayed. Nature appears to have developed her first nuclear reactors about two billion years ago, and she also devised a mechanism to safely confine the waste they produced deep beneath.
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