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.
