Two billion years ago— eons before humans developed the first commercial nuclear power plants in the 1950s— seventeen natural nuclear fission reactors operated in what is today known as Gabon in Western Africa [Figures 1 and 2]. The energy produced by these natural nuclear reactors was modest. The average power output of the Gabon reactors was about 100 kilowatts, which would power about 1,000 lightbulbs. As a comparison, commercial pressurized boiling water reactor nuclear power plants produce about 1,000 megawatts, which would power about ten million lightbulbs.
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Figure 1: The geology of the Franceville Basin. The natural nuclear reactors are located at Oklo and Bangombé. Other uranium deposits (which did not host natural nuclear reactors) are found at Boyindzi, Okélobondo, and Mikouloungou. Figure taken from Mossman et al., 2008.
Despite their modest power output, the Gabon nuclear reactors are remarkable because they spontaneously began operating around two billion years ago, and they continued to operate in a stable manner for up to one million years. Further, at the Gabon reactors many of the radioactive products of the nuclear fission have been safely contained for two billion years, providing evidence that long-term geologic storage of nuclear waste is feasible.
The possibility that natural nuclear reactors may have operated on the ancient Earth was first hypothesized by scientists in the 1950s, when commercial nuclear reactors were first being developed and becoming popular. Notably, in a 1956 paper Paul Kuroda theorized the conditions under which nuclear fission could spontaneously develop and be sustained.
Figure 2: Geologic cross-section of the Oklo and Okélobondo uranium deposits, showing the locations of the nuclear reactors. The last reactor (#17) is located at Bangombé, ~30 km southeast of Oklo. The nuclear reactors are found in the FA sandstone layer. Figure taken from Mossman et al., 2008.
These conditions are very similar to the conditions under which nuclear reactions are sustained in manmade nuclear reactors.
In manmade nuclear reactors, power is generated when uranium (or sometimes plutonium) atoms fission or break into parts, releasing nuclear energy. As a result of this fission, fast neutrons are produced. If slowed down by a moderating substance (typically water or graphite), these neutrons may induce other atoms to undergo fission. When carefully controlled, a self-sustaining “critical” reaction of nuclear fission can generate power for a long time—until the nuclear fuel becomes depleted of fissionable atoms. The energy produced by nuclear fission is generally used to heat water and produce steam, which turns large turbines that produce electricity.
Uranium is the most common fuel used in commercial nuclear power plants. Uranium has three isotopes: uranium-238, uranium-235, and uranium-234. Because of nuclear properties, uranium-235 is most likely to fission when bombarded with neutrons. However, on Earth today uranium-235 comprises only 0.720% of uranium while uranium-238 is the dominant isotope of uranium (99.275%) and uranium-234 is present only in trace amounts (0.006%). The isotopic distribution of uranium is remarkably uniform in Earth’s crust, so all uranium ore mined today contains about 0.720% uranium-235. In order to increase the efficiency of the nuclear chain reactions, uranium-235 is artificially enriched to approximately 3% before uranium is used as a fuel in nuclear power plants.
To control nuclear chain reactions in manmade reactors, water is used as both a moderator (something that slows down neutrons) and as a coolant. To control or shut down a nuclear chain reaction, control rods are used. These control rods consist of elements (such as silver, iridium, and cadmium) that are capable of absorbing neutrons without undergoing fission. Boron (another element very good at absorbing neutrons without undergoing fission) can also be added to water surrounding a nuclear reactor to moderate or shut down a nuclear reaction.
Thus, in manmade nuclear reactors the concentration of uranium, the abundance of uranium-235, and the presence of neutron moderators and absorbers are all carefully controlled. These same factors play a role in natural nuclear reactors.
There are four conditions which must be met in order for a stable natural nuclear reactor to develop:
1. The natural uranium ore must have a high uranium content and must have a thickness (at least ~2/3 of a meter) and geometry that increase the probability of spontaneous, natural fission in uranium-238 inducing a self-sustaining fission reaction in uranium-235.
2. The uranium must contain significant amount of fissionable uranium-235.
3. There must be a moderator, something that can slow down the neutrons produced when uranium fissions.
4. There must not be significant amounts of neutron-absorbing elements (such as silver or boron), which would inhibit a self-sustaining nuclear reaction, in the vicinity of the uranium.
Kuroda pointed out that the conditions necessary for a natural nuclear reactor to develop could have been present in ancient uranium deposits. Today, there are many concentrated uranium deposits, but—as you might be relieved to hear— it is impossible for nuclear fission to spontaneously develop. This is because the concentration of uranium-235 is too small (only 0.720% of uranium, as I mentioned above) for a self-sustaining fission reaction to be sustained. However, the relative proportions of uranium-238 and uranium-235 have been changing over the history of the Earth.
When the Earth was first formed, uranium-235 comprised more than 30% of uranium [Figure 3]. The proportion of uranium-235 relative to uranium-238 has been changing because isotopes of uranium are radioactive and decay to other elements over time. However, uranium-238 decays at a much slower rate than uranium-235, so uranium-235 has become more and more depleted (relative to uranium-238) over the Earth’s 4.54 billion year history. Billions of years ago, the abundance of uranium-235 in uranium ore was high enough for a self-sustaining fission reaction to develop. Two billion years ago, there would have been about 3.6% uranium-235 present in uranium ore— about the proportion of uranium-235 used in pressurized boiling water reactor nuclear power plants. So, in theory, an ancient (billions of years old) uranium deposit could have spontaneously developed a self-sustaining nuclear fission, assuming the uranium was concentrated enough, there was a substance (probably water) to act as a moderator, and there were not significant amounts of neutron-absorbing elements nearby.
Figure 3: Uranium-235 / uranium-238 in the Earth’s crust over time. The x-axis is in units of millions of years. When the Gabon natural nuclear reactors operated about 2 billion years ago, the Earth’s crust contained approximately 3.68% uranium-235. Figure taken from Gauthier-Lafaye and Weber, 2003.
Sixteen years later, in 1972, just such a natural nuclear reactor was discovered in Gabon. The French had been mining uranium in Gabon—their former colony— for use in nuclear power plants. During a routine isotopic measurement of uranium ore from Gabon, the French noticed something very strange: the uranium ore did not have a uranium-235 content of 0.720%. Rather, the uranium ore was anomalously depleted in uranium-235, containing only 0.717%. This may sound like a tiny variation, but this discrepancy was very alarming for the French nuclear officials. You see, uranium-235 in Earth’s crust (and even in moon rocks and in meteorites) varies very little from the average value of 0.720%. Since uranium-235 can be used to make nuclear bombs, it was very important to account for this “missing” uranium-235.
Fortunately, the nuclear officials and scientists eventually remembered the old publications of Kuroda and others, and they soon realized that the anomalous uranium from Gabon provided evidence of something extraordinary—the first natural nuclear reactor ever discovered. The uranium ore was depleted in uranium-235 because two billion years ago some of that uranium-235 had been used up in a natural nuclear reactor. Eventually, sixteen natural nuclear reactors were discovered in uranium mines at Oklo [Figure 1]. An additional seventeenth natural nuclear reactor was also discovered at Bangombé, located about 30 km to the southeast of Oklo.
The natural nuclear fission reactors in Gabon are unique— to date, no additional natural nuclear reactors have been discovered. Unfortunately for science, the sixteen natural nuclear reactors at Oklo have been destroyed, completely mined out for their rich uranium ore. Scientists only have limited uranium samples (often with sparse field notes) on which to conduct their study of these extraordinary nuclear reactors. In the late 1990s, there was danger that the last natural nuclear reactor at Bangombé would be mined as well. In 1997 scientist Francois Gauthier-Lafaye (and co-authors) wrote a plea to the journal Nature advocating that mining of the Bangombé uranium be stopped. They wrote,
The last known natural fission reactor on Earth is likely to be mined this year. Because these natural reactors are unique, at least one should be preserved for present and future research programs… All the reactors except one are located in the most important uranium deposit of Gabon’s Franceville basin, at Oklo… This deposit will be completely mined out soon, in 1998. Future work on these reactors will therefore have to rely on previously collected samples, many of which are poorly documented and are out of their geological context… Work is still possible, however, in a reactor located in the very small uranium deposit of Bangombe 30 km from Oklo. We propose that this unique, scientifically important deposit be preserved for present and future research. This deposit is no less unique, and certainly more irreplaceable, than the most valued specimens from the Moon and Mars.
Since the discovery of the Gabon natural nuclear reactors in 1972, scientists have been puzzling over why these reactors developed in Gabon two billion years ago and—seemingly— have developed at no other place or time on Earth. Scientists are still working to understand the Gabon reactors, but over the past forty years, they have managed to tease out some of the details of how these nuclear reactors operated and were preserved in the geologic record.
You might be wondering why natural nuclear reactors developed in uranium deposits only two billion years ago, when uranium-235 had already been depleted to less than 4% of uranium. Wouldn’t fission reactors have been even more likely to develop earlier in Earth’s history, when the uranium-235 levels were even higher? Remember that a high isotopic abundance of uranium-235 is just one of four conditions required for a natural nuclear reactor to develop. Another important condition is that uranium be concentrated. It turns out, no significant concentrations of uranium developed on Earth prior to about two billion years ago. The reason for this is simple: oxygen.
In most rocks on Earth, uranium is present only in trace quantities (parts per million or parts per billion). Uranium is generally concentrated by hydrothermal circulation, which picks up uranium and concentrates it in a new hydrothermal deposit. In order for this hydrothermal circulation to concentrate uranium, that uranium must be soluble (able to be picked up in water). However, uranium solubility is a little tricky. When uranium is in its reduced form (U4+), uranium tends to form very stable compounds that are not easily brought into solution. However, when uranium is in its oxidized form (U6+), uranium easily forms soluble complexes. There was very little oxygen in Earth’s very early atmosphere. So, it would have been very difficult to concentrate a significant amount of uranium since there was no oxygen to transform uranium into its soluble forms.
However, starting around 2.4 billion years ago, there was an event called the “Great Oxidation Event” during which the levels of oxygen in the atmosphere rose significantly, from <1% to ≥15%. This significant rise in atmospheric oxygen was a result of photosynthetic cyanobacteria producing oxygen. For awhile, the oxygen produced by these bacteria was taken up by minerals which became oxidized. However, when these minerals became saturated in oxygen, this oxygen began to accumulate in the atmosphere. This increase of atmospheric oxygen allowed uranium to become mobile and to be concentrated through hydrothermal circulation.
In Gabon rich uranium deposits formed about two billion years ago in a marine sandstone layer in the Franceville Basin [Figure 2]. The lower part of this sandstone layer originally contained many small bits of uranium-bearing minerals (monazite, thorite, probably uraninite). These minerals were dispersed until the sandstone became infiltrated with oxidizing waters around two billion years ago. These oxidizing waters dissolved the uranium-bearing minerals and concentrated the uranium in several deposits towards the top of the sandstone layer. The uranium actually became extraordinarily well-concentrated. Fission of uranium could have begun when the uranium concentration reached 10%; the Gabon uranium deposits in which natural nuclear reactors developed contained about 25% to 60% uranium.
Thus, two billion years ago in Gabon two of the four conditions for the development of a natural nuclear fission reactor were met: there were significant concentrations of uranium, and this uranium still contained a significant amount of highly-fissionable uranium-235. The other two conditions were also met. Water was able to percolate into the permeable sandstone containing the uranium deposits, and this water acted as the neutron moderator. There were also no significant quantities of neutron-absorbing elements to inhibit the self-sustaining fission reaction. All of this provided the perfect recipe for a natural nuclear fission reactor.
The Gabon natural nuclear reactors operated for several hundred thousand years.The reactors likely switched on and off at regular intervals. The nuclear fission began, moderated by water, and continued until all available water boiled away as a result of nuclear heat. The reactions could not begin again until new water infiltrated the reactor. This on-and-off behavior of the reactors probably operated over a timescale of a few hours, analogous to the way in which geysers erupt periodically as a result of groundwater recharge. Possibly because of this periodic on-and-off behavior, the Gabon natural nuclear reactors were extremely stable. There was not a single melt-down; the reactors operated in a stable fashion for up to 1 million years. Eventually, the fissionable uranium-235 was depleted, and the Gabon natural nuclear reactors shut down.
The long-term preservation of the Gabon natural nuclear reactors is perhaps even more remarkable than the reactors themselves. These nuclear reactors have survived two billion years of geologic time. The preservation of the Gabon reactors is a result of two factors: the long-term stability of the African craton, and the isolation of the uranium deposits from oxidizing groundwater. The natural nuclear reactors in Gabon seem to have been largely protected by enveloping carbonaceous substances and clay, which created and maintained reducing (low oxygen) conditions which largely inhibited the movement of uranium and other radioactive products of nuclear fission.
Perhaps natural nuclear reactors operated in several other places on Earth two billion years ago. Perhaps we haven’t yet found evidence of other natural nuclear reactors, or perhaps the radioactive remains of other natural nuclear reactors have long since been eroded or oxidized and dissolved. As of today, however, the Gabon natural nuclear reactors remain “unique, and certainly more irreplaceable, than the most valued specimens from the Moon and Mars.” Since the Gabon reactors were so stable, operated over such a long time, and have been preserved for two billion years, scientific study of these unique natural reactors provides important insights relevant to anthropogenic nuclear power and nuclear waste storage. Mother Nature, it seems, knows how to operate a nuclear reactor.
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