The first nuclear reactor on Earth was built 2 billion years ago. Who was responsible for its construction?

Long before humans constructed their first nuclear reactors, nature independently created the very first one. This occurred at a time when only microbes dominated the planet, as reported by IFLScience.

In 1972, at a nuclear fuel processing plant in Pierrelatte, France, one of the physicists was analyzing uranium samples. He noticed something unusual in the data he received.

Typically, three different isotopes are found in uranium ore deposits: uranium 238, uranium 234, and uranium 235. Uranium 238 is the most common, while uranium 234 is the rarest.

Uranium 235 accounts for about 0.72% of uranium deposits and is the most sought after. If it is enriched to over 3%, the isotope can be used to create a sustainable nuclear reaction.

In samples from Oklo (Africa), the proportion of isotope 235 was found to be 0.717%. At first glance, this difference may seem minor, but it is quite peculiar.

"All natural uranium today contains only 0.720% uranium 235. You would get the same results when checking rocks from the Earth's crust, even lunar rocks or meteorites. However, a piece of rock from Oklo contained only 0.717%," explains the International Atomic Energy Agency (IAEA).

Further investigations revealed that other deposits in the region contained even less of the isotope, around 0.4%. Initially, scientists speculated that the deposit somehow underwent a stable nuclear fission reaction. However, they later discovered that the uranium at this location had gone through a stable natural fission reaction over 2 billion years ago.

"The study showed that the deficiency of 235U in Oklo could have arisen from isotopic fractionation in the Earth's crust or during a natural chain reaction. Soon, analyses confirmed that it was indeed a chain reaction, indicated by the anomalous presence of rare earth isotopes," stated a report from the U.S. Geological Survey.

Nowadays, conditions for such reactions are nearly nonexistent, as the concentration of uranium 235 in the area was much higher in the past. This site also needed to be saturated with groundwater to sustain the reaction, just like in modern nuclear reactors that slow down neutrons produced during fission.

As the water heated up and evaporated during the process, neutrons were not slowed down and "escaped" without further reaction. The fission stopped before the water cooled and seeped into the deposits deep enough for the fission process to restart.

Ultimately, after thousands of years, the world's first nuclear reactor slowly came to a halt.

As a reminder, physicists have demonstrated what happens inside a thermonuclear reactor. With stunning visualizations, one can see how thermonuclear fusion occurs inside a tokamak.