Imagine discovering that the first nuclear reactor was not built by humans, but by nature itself, two billion years ago.
That’s exactly what happened in 1972, when a team of French scientists analyzed a sample of uranium ore from a mine in Oklo, Gabon, and found that it contained a lower than normal proportion of uranium-235.
Uranium-235 is the isotope that can undergo nuclear fission, the process of splitting atoms to release energy. The scientists realized that the only possible explanation for this anomaly was that the uranium ore was part of a natural nuclear reactor that operated in the distant past.
“After more investigation, including on-site investigation, they discovered that the uranium ore had gone through fission on its own,” said Ludovic Ferrière, curator of the rock collection at the Natural History Museum in Vienna, where some of the remarkable rock will be presented to the public in 2019. “There was no other explanation.”
How did this natural nuclear reactor work? And what can we learn from it today?
The Oklo reactor was one of seventeen natural reactors that emerged in a region of Gabon about two billion years ago, when the concentration of uranium-235 in the Earth’s crust was much higher than today.
The reactors were located in a layer of sandstone that contained rich deposits of uranium ore. The sandstone also acted as a natural filter for groundwater, which played a crucial role in moderating the nuclear reaction.
Water is a good moderator because it slows down the neutrons released by fission, making them more likely to cause further fission in other uranium atoms. This creates a chain reaction that sustains nuclear energy generation.
“Just like in a man-made light-water nuclear reactor, without anything to slow or moderate the neutrons, the fission reactions simply stop,” said Peter Woods, team leader in charge of uranium production at the IAEA. “The water acted as a moderator in Oklo, absorbing the neutrons and controlling the chain reaction.”
However, if the water level is too high, it can also absorb too many neutrons and stop the reaction. This is what happened in Oklo, where the groundwater level fluctuated over time, creating a natural feedback mechanism that regulated the reactor’s output.
The Oklo reactor operated intermittently for hundreds of thousands of years, producing about 100 kilowatts of power, enough to light 1,000 light bulbs.
The reactor’s power varied depending on the water level, producing current pulses on average every three hours. The reactor also generated several radioactive byproducts, such as plutonium, neodymium, ruthenium and xenon.
Some of these byproducts were used as evidence to confirm the existence of the natural reactor, as they matched the expected fission products of a uranium-235 fuel.
The Oklo reactor is not only an incredible example of nature’s ingenuity, but also a valuable source of information for modern nuclear science and engineering. One of the most important lessons we can learn from Oklo is how to safely store nuclear waste for long periods of time.
The radioactive byproducts of the natural reactor have been remarkably well confined within the original ore deposit for two billion years, with minimal leakage or migration into the environment. This shows that geological formations can provide effective barriers for isolating nuclear waste from the biosphere.
Another lesson we can learn from Oklo is how to design more efficient and sustainable nuclear reactors for the future. The natural reactor had a very high fuel utilization rate, meaning it extracted the most energy from the uranium ore before it became depleted.
The natural reactor also had a very low impact on the environment, as it released no greenhouse gases or other pollutants. Furthermore, the natural reactor was self-regulating and self-stabilizing, without the need for human intervention or control systems.
The Oklo reactor reminds us that nature has experimented with nuclear energy long before we did, and that we can benefit from studying its secrets and solutions.
