The first nuclear reactor? Nature created one two billion years ago

In Gabon, two billion years ago, a natural nuclear reactor was formed: a unique event that offers precious clues about the history of our planet and its incredible capabilities

The first thing that comes into our minds when we think about nuclear reactors are big human-engineered constructions, symbolic of technological advancement or debates on environmental impact. But what to do if nature itself had already designed a fully working nuclear reactor billions of years before humanity grasped the principles? This story takes us right to the heart of Africa-in Gabon-where a uranium deposit sent shockwaves around the globe.

An anomaly in the rocks

In 1972, a physicist in France analyzed a set of uranium samples from the Pirlat nuclear plant and detected an anomaly. Natural uranium has three basic isotopes: uranium-238, uranium-235, and uranium-234. The first two isotopes are the rarest among these, taking only 0.72% of total reserves, and this very isotope takes part in nuclear reactions. Nevertheless, this sample showed a decreased rate to 0.717% for uranium-235. An infinitesimal difference, yet one sufficient to ring an alarm bell.

The traces of uranium came from the Oklo deposit in Gabon. As the scientists dug deeper, it became clear that this anomaly was not an accident. It was actually the result of a naturally occurring chain reaction that took place about two billion years ago. This unique case in Earth’s geological history was the result of an exceptional combination of circumstances.

As the United States Geological Survey reported, the evidence was unmistakable: from the odd distribution of uranium isotopes to the presence of noble gas isotopes such as krypton and xenon—absolute telltales of nuclear fission.

How the natural nuclear reactor worked

Two billion years ago, the abundance of uranium-235 was significantly higher than today, around 3%, sufficient to allow spontaneous fission. The key element enabling the process’s stability was groundwater, which acted as a natural moderator, slowing down neutrons much like in modern reactors.

When the water heated up and evaporated, the reaction shut off, and when the temperatures dropped and water flowed again, the reaction turned back on. This self-regulating cycle went on for hundreds of thousands of years and produced an estimated power output of about 100 kilowatts. Eventually, the natural reactor shut down and gradually dismantled itself.

Lessons from nature’s nuclear experiment

The discovery of the Oklo natural reactor is a very unique chapter in geology but also one that carries an important lesson for today. It shows how nature, with its complex chemical and physical interplay, can achieve what continues to amaze science.

For researchers, this is a natural laboratory in which to study processes that might otherwise be difficult or impossible to reproduce. For the rest of us, it’s a reminder that our planet still holds extraordinary secrets, waiting to be uncovered.

Source: Physical Review