As you look at the results in front of you, you start to wonder if you’re going insane.
Have you just made some silly mistake that’s given you totally misleading test results?
Because what you appear to have discovered makes no sense at all.
A concentration of uranium this high?
It can’t be possible.
You work at a uranium enrichment plant in France - you deal with uranium day in, day
out, and you know just about everything there is to know.
Yet you’ve never seen anything quite like this.
Your task for the day had been to carry out a series of checks on a natural uranium sample.
Nothing out of the ordinary for somebody who works in a uranium plant.
The only reason you were carrying out the checks was a slight isotopic anomaly in some
samples from Gabon, and nobody suspected that there wouldn’t be a simple explanation.
But here you are looking at a result that contradicts everything you thought you knew.
These results suggest the uranium was involved in a nuclear reaction.
But how could a nuclear reaction have happened in a remote mine in Gabon?
Maybe it was contamination.
But, if not, it could mean a nuclear reaction had happened naturally.
That would rewrite history and change everything you thought you knew…
See, a nuclear reaction is no small matter.
Once upon a time, scientists weren’t even sure if such a thing was possible.
You remember when the first nuclear reactor was created, back in the 1940s.
It was man-made, built by a physicist called Enrico Fermi.
You were just a child at the time, so you don’t remember the events well, but the
whole world was in awe at the groundbreaking discovery.
Later, as you went on to study physics, you’d realize what a monumental achievement it was.
You learned how Fermi had been just starting out his career in physics when scientists
discovered nuclear fission for the first time, and he began to ponder the implications.
Maybe this mysterious process could be used on purpose by humans to create energy…
To investigate this theory, he carried out some experiments with uranium, an essential
component of nuclear fission.
All this would eventually lead to setting up the first nuclear chain reaction.
At least, so he thought at the time…
In 1942, Fermi and his team assembled a reactor built of 45,000 graphite blocks and wood,
materials that acted as the reactor’s neutron moderator.
A powder made of uranium oxide was poured into the blocks to set the right conditions
for a reaction.
And finally, a control rod made of cadmium was used to control the reaction by absorbing
neutrons and acting as a kind of break.
The reactor was named Chicago Pile-1.
One day later that year, Fermi attempted the experiment.
The trick was basically to get to the level of criticality needed to guarantee self-sustaining
nuclear fission.
Nuclear fission happens when an atom splits in two parts and releases energy.
Atoms contain protons and neutrons in their central nucleus, but when the nucleus splits
during fission, its resulting pieces have less combined mass than the original nucleus.
This missing mass becomes nuclear energy, which has great potential as a power source.
Once the reaction becomes self-sustaining, it can continue with no intervention, because
the neutrons created through fission give off enough energy to sustain another reaction.
For this to be possible, the neutron multiplication factor has to be high enough.
Basically, it was a highly complex experiment.
Nobody had attempted it before, and nobody was sure it would work.
So many precise factors had to line up perfectly – no wonder the idea of this happening in
nature seemed impossible.
But the experiment was successful.
It was monumental within your discipline and for all of humankind.
Fermi proved nuclear energy could generate power, and his model was used as a baseline
for large-scale nuclear reactors built afterward.
Everyone thought it was the first nuclear reactor ever.
How could something that was discovered as recently as 1942 have happened without the
intervention of man so long ago?
Yet this is what the results are indicating.
Your colleagues are incredulous when you tell them, but together you carefully check over
the results and consider the possibilities – it was possible the data could be misleading
due to contamination.
But already, your mind was swimming.
What if this genuinely had happened naturally?
How could it have taken place?
How old would the reactor have to be?
It was all so crazy – you hadn’t even been looking for a nuclear reactor in the
first place!
But you definitely got more than you’d bargained for.
You realized that there was more of the isotope uranium-235 than there should be – more
than there had been in any other uranium sample, ever.
Natural uranium, which is taken from the crust of the Earth or rocks from the moon, is made
up of only around 0.720% of U-235.
But rock in Oklo only contained 0.717%.
It might sound like a pretty minor difference, but it’s actually incredibly significant.
The concentration of U-235 was supposed to be so constant throughout all natural sources
in the solar system that it’s commonly used to calibrate devices.
How come this piece of uranium was breaking an established natural law?
As well as contradicting science, the finding meant that this uranium was the high-grade,
fissile sort.
And, since you needed to be sure that none of the uranium handled is used for weapons
purposes, you needed to carry out an investigation.
It was definitely weird, but whatever.
You assumed it had to be some kind of artificial contamination.
Maybe this uranium had been contaminated with depleted uranium, which has less U-235, at
some point during the production process.
This was sounding slightly more likely.
You decided to take a look.
First, you traced the process of sourcing the uranium back in time.
It had been through a long journey to get where you are now.
From the Oklo mine in Gabon to a mill nearby to a processing plant in France, and now here,
in this enrichment plant.
Luckily, samples of the ore were kept along each stage for the very purpose of carrying
out investigations like this one.
It was easy to check the properties and figure out if there had been any contamination.
One by one, you checked them over.
But the results were strange – it turned out that the samples at every stage had a
lower level of uranium-235 than they should.
The ore you’d examined wasn’t just an anomaly.
But why?
You were really scratching your head now.
Maybe other researchers had already split the isotopes during artificial fission – a
nuclear chain reaction similar to the one Fermi carried out.
Then again, that seemed unlikely considering the remoteness of the mine in Oklo.
After investigating further, it was clear that wasn’t the case.
If so, there would be depleted uranium missing.
But there wasn’t.
Another theory was that a bomb had exploded in Africa without anyone noticing.
Since bombs cause nuclear reactions, this would have changed the ratio of U-235 and
U-238, the other isotope present in uranium.
Yet the readings were found to be localized to ore in the mine, so you had to dismiss
this idea too.
The mystery grew.
Unless a natural reaction really had happened.
So, you looked for fission products, the isotopes of elements created during nuclear fission.
Sure enough, there they were.
The uranium had somehow become a natural reactor that went critical, burned up a portion of
its nuclear fuel, and shut down.
There was only one question left: how?
You weren’t just assuming that any nuclear reaction had to be man made because you were
a stubborn scientist or a fanboy of Fermi.
There were good, scientific principles to suggest why natural nuclear reactors should
be impossible.
So, to answer the question of how, we need to take a deep dive into the science…
There’s been plenty of talk of uranium-235.
But what even is it?
Well, uranium has two principal isotopes: U-235 and U-238.
The ratio of these two in uranium ore should be the same everywhere.
But that’s not to say that the ratio has always been the same.
Uranium decays over time, and both the isotopes decay differently.
Now, U-235 has a shorter half-life, which means its concentration within uranium decreases
with time – there was a higher concentration in the past than there is now.
For U-238, the opposite is true – its concentration increases over time.
So what?
Well, this alone doesn’t prove that a natural nuclear reaction took place.
But it does give a clue.
The ratio of U-238 and U-235 that exists now doesn’t make it possible for a nuclear reaction
to take place.
But if the ratio was different, it could be possible.
And, since the isotopes decay at different rates, once upon a time the ratio would have
been different.
It could have been optimal.
Two billion years ago, the ratio of uranium-235 to uranium-238 was about 3%, which incidentally
is perfect for a light-water-moderated chain reaction.
So, the first hint was there that the reaction happened a very, very long time ago.
And this wasn’t the only thing.
Other conditions for a nuclear reaction were met in the Oklo mine.
Firstly, there was a high concentration of uranium in the ore body (10% or more), and
the ore was concentrated in seams at least half a meter thick.
During a sustained chain reaction, the fissioning of a uranium-235 nucleus emits two and a half
neutrons, one of which induces fission in another nucleus, whilst the others are absorbed.
The ore seam therefore needs to be relatively thick to allow for this absorption.
It sounds complicated, but trust me.
Secondly, there was water in the mine.
This was perhaps the most important part of all.
For a nuclear chain reaction to happen and be self-sustaining, it needs a moderator,
and water fits the role perfectly.
Water slows down neutrons and makes controlled fission possible – without this, the atoms
wouldn’t have split.
But it can’t just be any kind of water.
There’s both heavy water and so-called ordinary water in the world.
Heavy water contains a type of hydrogen called deuterium, whereas ordinary water is made
of protium.
And it also contains unique properties that allow it to act as a moderator.
Nowadays, heavy water is needed for a nuclear reaction to happen, because of the ratio of
U-235 and U-238 found in uranium now.
But remember how the ratio changes over time?
Well, two billion years ago, there was more U-235, which means ordinary water could have
moderated the reaction.
Amazingly, every single condition for a nuclear reaction was met.
Theoretically, a fission chain reaction could have been triggered in the uranium deposits,
all those years ago.
But at the same time, it seemed impossible to prove it.
You couldn’t just go to the mine and check the conditions for the reaction, because there
was no guarantee that the conditions were the same now as they had been all that time
ago.
Plus, given that the reaction happened billions of years ago, there would surely be no traces
left.
The earth moves over that large a time span, especially uranium in the presence of circulating
water, like that found in the mine.
The only way it could be proven would be through a fossilized nuclear reactor.
It seemed like a long shot, but it was worth a go.
And after investigating the mine, this is exactly what you found.
All the conditions had been met, they were still in place now, and there was a fossilized
nuclear reactor.
Incredible.
Even better, you found a footprint of fission products in the ore that was there too.
So, what did all this mean?
What had happened in the Oklo mine?
Picking apart the evidence, you were able to paint a fuller picture.
It seemed like the fission reaction had continued on and off for hundreds of thousands of years,
sustaining itself fully throughout this time.
Then, it eventually stopped reacting and shut down.
This probably happened around one billion years ago.
To put that into context, humans have only been around for about 200,000 years.
Jellyfish, one of the world’s oldest species, have been around for maybe 500 million years.
Even earth itself is only estimated to be four to five billion years old.
Basically, this nuclear reactor is only half as old as the very planet we live on.
It’s hard to wrap your head around!
But why did this happen in Oklo, and has it ever happened anywhere else?
Oklo may have had the right conditions, but it’s hard to know why the uranium deposit
went critical there.
One reason could be the lack of elements in the mine that prevent a reaction taking place,
like lead and cadmium.
Another possibility is its remoteness.
Whilst being remote doesn’t make a nuclear reaction more likely, it does improve the
likelihood of the byproducts being preserved.
As for whether there have been other natural reactors, there’s no way to be certain.
16 sites of nuclear fission reaction have been found in Oklo, but none elsewhere in
the world.
That’s not to say that there aren’t anymore – but it’s pretty unlikely that conditions
would be remote enough to have preserved the evidence anywhere else.
Experts suspect there have been other natural reactors like this that were destroyed by
geological processes or eroded away.
And what does all this mean, anyway?
It might be kind of cool that such an ancient nuclear reactor has been found, but does it
have relevance to anything real?
Well, it certainly provides some insights into waste management.
Whilst active, Oklo probably depleted around ten tons of uranium ore, yet the waste left
was so insignificant that visiting researchers didn’t even notice.
Looking at what happened gives some clues about how to deal with the waste produced
by nuclear power plants, which is essential if nuclear ever becomes a conventional source
of energy.
Now go check out our videos about a US nuclear accident more powerful than Hiroshima and
Russia’s floating nuclear power plant.