Is this the most natural form of energy generation?

Examining nature's geological power stations

“When you have eliminated the impossible, whatever remains, however improbable, must be the truth” - Sherlock Holmes

When most people think of natural energy sources, they think of solar and wind power because they are powered by the elements. I find myself constantly refuting this argument by pointing out that harnessing the power of the wind and sun still requires human intervention, including mining, resources, construction, and infrastructure. Although the idea that one thing is more ‘natural’ than another is spurious and illogical, it seems to be human nature to associate some things with being natural and, therefore, more positive, healthy, or desirable, so let’s humour this idea and consider what would actually be classed as ‘natural’ when considering sources of energy generation.

The conclusion might surprise you.

To do this, we have to go back two billion years, to a time well before humans invented conventional nuclear power plants, to when scientists discovered evidence of 19 natural nuclear fission reactors operating in the Oklo, Okelobondo, and Bangombe mines in Western Equatorial Africa in the Gabon Republic. You read that right — nuclear power was invented by nature.

Inside the Oklo mine in Gabon

A rocky discovery 

In 1972, during a routine analysis of uranium ore in France, French physicist and high commissioner for France's Atomic Energy Commission Francis Perrin came across a dark piece of radioactive natural uranium ore that had been extracted from Oklo. Gabon had been a French colony for many years, so tensions were high, and terrorism was a serious potential threat to the French at the time.

Perrin found that the material contained a lower proportion of uranium-235, which meant that it was likely that the ore had gone through some form of artificial fission - as in, U-235 isotopes had been split in a nuclear chain reaction. As a nuclear weapons expert, Perrin was worried that terrorists may have stolen the uranium for nefarious purposes. The CEA thought that there was enough uranium at Oklo for six bombs to have been made. 

When the news got out about the remnants of fission in the mines, it was such a shocking discovery that people reacted with all manner of panicked theories. Some even argued that Oklo was proof that ancient aliens had once landed on Earth and left behind uranium tracks at Oklo.

However, once scientists looked closer, they saw that the lower percentages of uranium-235 in the rock were surrounded by dark patches, as if the rocks had been scorched. It didn’t seem possible that humans could have managed this without leaving behind evidence of their exploits. What could produce so much heat in an underground, typically cool mine?

The Oklo mining site. Source

After further testing, Perrin and his peers concluded that - amazingly - Mother Nature was behind everything. Fission had occurred in the mines over two billion years ago, and the fission process produces heat, which explains the scorched black rocks the scientists found. The French had made an incredible discovery: the first evidence of a natural nuclear reactor. But how had this happened without human interference?

At the time, Oklo was a mudflat of soft clay soil that contained various minerals, including uranium, and the site flooded regularly. The water collected in pools which contained algae, or pond scum. The algae would absorb the uranium water, which concentrated it, and when the algae died, it piled up, which concentrated the uranium even further until it eventually reached the critical mass needed to start a chain reaction. For a nuclear chain reaction to take place and continue, there needs to be a moderator to slow the neutrons down. In this case, the moderator was water. The water also slowed neutrons down enough to allow a chain reaction to start, and there was just enough uranium-235 to keep the chain reaction going.

When this critical reaction is controlled, it can generate power for many years. However, the water supply wasn’t steady, so the natural reactors would have stopped and started numerous times, operating for a few years to hundreds of thousands of years. The total period over which the reactors operated is thought to be about a million years.

The average power output of the Gabon reactors was small - around 100 kilowatts, which could power around 1,000 lightbulbs or 225 computers. By comparison, nuclear power plants usually produce over 1,600 megawatts of power, enough to power entire cities.

While active, the natural reactor generated fission waste byproducts similar to those produced by modern nuclear reactors at power plants. This provided some useful evidence for the scientists, who found that the radioactive waste products created by this natural process, including those with million year half lives, have decayed away. The byproducts have also barely moved - according to the US Department of Energy, the plutonium “has moved less than 10 feet from where it was formed almost two billion years ago.”

This means that when the Oklo reactor was discovered in 1972, the fission products had been harmlessly lying in the same place for around a billion years.

Also, in the hundreds of thousands of years it has operated as a nuclear reactor, Oklo has never had a meltdown or explosion. Scientists found that “the combination of aluminium phosphate grains to trap radioactive materials and the groundwater to regulate the reaction allowed for an extremely safe reactor.” Mother Nature knows best.

Earth has almost certainly created other natural reactors, but we haven’t discovered them yet, and the theory is that geological processes have destroyed or eroded evidence of their existence. Regardless, the evidence we have from Gabon is conclusive in demonstrating that nuclear fission is the most natural form of energy generation.

A sample of this rare and wild piece of science and history – nature’s nuclear reactor - is displayed in Vienna’s Natural History Museum.

So next time someone tells you that solar and wind are the only ‘natural’ forms of energy generation, tell them about the natural reactors in Gabon. I’ve yet to hear about solar panels and wind turbines sprouting up naturally and generating electricity without human intervention anywhere in the world in the history of our planet. The blunt truth is that nature created fission well before humans were capable of building nuclear reactors. If that isn’t a clear definition of ‘natural’ energy, I don’t know what is.