Deep in the African continent lies a nuclear conundrum.
Cast your mind back to 1956, the Cold War had been silently raging for a decade and Elvis Presley’s Blue Suede Shoes is on the wireless. Every major power in the world wants to get there hands on some precious Uranium 235 to join the Nuclear Powers’ rank. Luckily for France, they controlled the African country of Gabon who’s Oklo region was rich in Uranium ore. But, the Oklo mined Uranium looked very weird. It already looked like it had been in a reactor! Had someone beaten them to it and used their Uranium?! No, in fact, they had discovered one of the most unique geological structures ever found, a two billion-year-old natural nuclear reactor.
There are several different Uranium types, also known as isotopes, but the one we want for nuclear reactors and bombs is Uranium 235 (²³⁵U). This isotope has a half-life of 700 million years, which means that every 700 million years, half of the ²³⁵U will naturally radioactively decay into Thorium 231. But by tickling the ²³⁵U with a slow-moving neutron we can make it decay instantly and cause a chain reaction, also known as a fission reaction. This is what makes ²³⁵U so special.
Luckily there is lots of naturally occurring neutron radiation to start our reaction off, but these move far to fast to interact with a ²³⁵U atom, they need to be slowed down. By placing the ²³⁵U in a control material, like water, these neutrons are slowed down enough to be absorbed by the ²³⁵U. This turns it into a new isotope called ²³⁶U. This new isotope is highly unstable and quickly decays into Baryum 141 (¹⁴¹B), Krypton 92 (⁹²Kr) and three high energy neutrons. These neutrons once slowed down, can cause three more ²³⁵U atoms to decay and so on and so causing a runaway exponential nuclear fission chain reaction.
This chain reaction is what powers nuclear power plants and bombs, a simple domino effect of radioactivity.
But just like carbon dating, you can estimate the ratio of ²³⁵U there should be in Uranium ore if you know how long ago the Uranium was made. For example, if the ore is 700 million years old, you would expect there to be half the ²³⁵U there compared to when it was made.
All Uranium we have an Earth was formed at the same time, this is when the Star that predated our Sun went supernova, some six billion years ago. In fact, you can use Uranium to estimate the age of the galaxy using this method. But this means all Uranium ore on Earth should currently have a ²³⁵U concentration of 0.72%. But the French scientists actually found a concentration of 0.717%.
This may sound like a small difference in concentration, but Oklo had a lot of Uranium! So the equivalent of 200 kg of ²³⁵U just disappeared right from under their noses. That is enough to build six atomic bombs!
As you can imagine, eyebrows were raised. At the time, the global political climate hinged around ²³⁵U as the Soviets and United States ramped up their nuclear stockpiles. Smaller countries like France and the United Kingdom had to build their own nuclear weapons to hold any international power. The French were worried someone had stolen their source of ²³⁵U, which could have caused a huge international upset, depending on who had it.
But on closer inspection, this ore had huge amounts of ²³⁵U decay byproducts, like the ⁹²Kr, ¹⁴¹B and their daughter particles (both ⁹²Kr and ¹⁴¹B are themselves radioactive and decay). This suggested that this ²³⁵U had already been in a reactor and then placed back in the ground…
It made no sense!
But this was hypothesised years before they even started digging the Oklo mine. A geological formation that acts as a naturally regulated nuclear reactor. After some more surveys of the area, they discovered that Oklo matched all of the formation criteria. This is the first discovered natural nuclear reactor.
So how does a naturally occurring reactor work? And how come it didn’t go supercritical and explode?
This reactor was active a long time ago, some two billion years ago in the Paleoproterozoic era. Back then, multicellular life was relatively new. This is so long ago that the ²³⁵U concentration would have been around 3%, which is the same concentration as the nuclear material in modern-day reactors.
There is just one problem. Without a control to slow down the neutrons, criticality can’t be achieved, so no chain reaction happens. This is because neutrons from natural ²³⁵U decay simply fly pass the other ²³⁵U atoms. We need something to slow down the neutrons so they can attach to the ²³⁵U and start a chain reaction, making the reactor go critical.
But Oklo did have this neutron control in the form of a water table. The Uranium ore was submerged in a subterranean waterway that consistently flowed through this reactor. This water slowed down the neutrons, making the ore go critical and sustain an ongoing chain reaction.
Now, if the water just stayed there, the reaction would have ramped up and gone supercritical! This is where the chain reaction runs away, and more and more ²³⁵U split at an exponential rate. This is what happens in a nuclear bomb!
But Oklo would never go supercritical because the water not only started the chain reaction but had the means to also stop it, effectively regulating the reactor.
In the image above, you can see a cross-section of the Oklo reactor. When the water flows through the reactor, it slows down the neutrons making the ore go critical and heat up. Then after about half an hour, the water would get so hot that it would boil away, leaving the reactor with no way of slowing down the neutrons, stopping the reaction. As the reactor cooled and water started to flow through it, the reaction would start again, and the Oklo reactor stayed like this for hundreds of thousands of years until the ²³⁵U concentration was too low to go critical.
So because of this water control, Oklo was never going to have a nuclear meltdown or explosion. But it did release a surprising amount of energy. Averaged out, the reactor had an output of 100 kW of heat energy. This is the same energy as 1000 incandescent lightbulbs or slightly more power than a Delorean at full throttle. Only for several hundred thousand years!
It just goes to show you that nature is far weirder than anyone thought possible. Now we know these exist, it’s amazing to think that other places in the Universe could host such complex geology. It isn’t a far leap to imagine a distant planet with a natural nuclear reactor of its’ own powering hydrothermal vent like structures. Structures like this could host primitive radiation-resistant bacteria-like life that lives off the mineral-rich heated water spewing out of the reactor. Imagine that, nuclear-powered bacteria!