Nature's very own nuclear reactor

Bethany Rothwell looks back on how Mother Nature beat nuclear scientists to the punch

Mankind has been harnessing the power produced by nuclear fission since the 1950’s. While this may seem like a man-made concept, it turns out that nature got there billions of years before us! In 1972, it was discovered that a mine in Oklo, Gabon, is home to 17 natural nuclear fission reactor sites that operated nearly two billion years ago.

The discovery was made when scientists found that the mine contained significantly less fissile uranium than expected during a routine mass spectrometry. With this being such a rare and useful fuel, they were keen to know where the missing 200 kg (enough to make 6 nuclear bombs) had gone. As it turned out, the uranium had been used up in natural fission reactions over the course of several hundred thousand years.

Nuclear fission occurs when an unstable nucleus splits into smaller parts, releasing huge amounts of energy. The most common fuel is the isotope uranium-235. Upon impact by a neutron, a uranium-235 nucleus fissions to produce two lighter nuclei, plus a few more neutrons; these then go on to cause further fissions in a chain reaction under three main conditions.

One: Enough concentrated uranium is present to allow for a self-sustaining reaction.

Two: There is a significant abundance of uranium-235 within the sample.

Three: A moderator, such as water, is available to slow down the neutrons—if they’re travelling too fast, it’s unlikely they’ll be absorbed.

Over the decades, significant research has ensured nuclear reactors meet these conditions perfectly—this shows how remarkable it is that Mother Nature did it all by herself! So how was this achieved?

When the Earth was formed, there were no significant uranium concentrations—the sandstone in Oklo contained only tiny samples dispersed amongst its layers. When the atmosphere became saturated with oxygen two billion years ago, that uranium was transformed into its soluble oxide. Once water could seep through the sandstone, the samples were dissolved and became mobile, resulting in deposits concentrated enough to satisfy the first condition.

A natural fission reactor couldn’t operate today, due to the insufficient natural uranium-235 concentration of 0.72%; for uranium-235 to be used as a fuel source, power stations often enrich it to achieve a concentration of 2-4%. However, when the Earth was younger, this abundance was higher—the relatively short half-life of uranium-235 has caused its fast decay. When the Oklo reactor began fissioning all that time ago, the concentration was a healthy 3.6%, thus fulfilling the second condition perfectly.

The final condition was met by oxygen-bearing water that acted as a suitable moderator, allowing fission to occur until the reaction exotherm caused it to boil away. Fission could only then begin again, once the water had cooled enough to flow back into the reactor. This continuous, stable cycle allowed the reactor to continue producing energy for such a long period (similar to the negative feedback mechanism used to keep modern reactors safe).

It’s clear that this natural reactor was remarkable. Perhaps even more astounding is its ability to safely contain the radioactive products underneath Oklo for nearly two billion years. With investigations on the effects of nuclear waste disposal on its surrounding areas taking place, Oklo mine emerges as the best long-term study scientists could have hoped for. There is no doubt that mankind can learn a lot from this incredible discovery—it seems that Mother Nature still has lots to teach.

From Issue 14