When was the first nuclear reactor created? You probably think it was Enrico Fermi’s CP-1 pile built under the bleachers at the University of Chicago in 1942. However, you’d be off by — oh — about 2 billion years.
The first reactors formed naturally about 2 billion years ago in what is now Gabon in West Africa. This required several things coming together: natural uranium deposits, just the right geology in the area, and a certain time in the life of the uranium. This happened 17 different times, and the average output of these natural reactors is estimated at about 100 kilowatts — a far cry from a modern human-created reactor that can reach hundreds or thousands of megawatts.
The reactors operated for about a million years before they spent their fuel. Nuclear waste? Yep, but it is safely contained underground and has been for 2 billion years.
The Basics of Fission Reactors
Any fission reactor basically works the same. An unstable atom like uranium or plutonium breaks apart, creating fast neutrons. There’s a chance some of those neutrons will strike nearby unstable atoms, causing them to break apart too. However, because these neutrons are fast, statistically this won’t happen very often. However, if a moderating substance like water or graphite slows down the neutrons, they have more time to interact with other unstable atoms, so more atoms break down.
If you balance it right, the atoms breaking apart will cause a chain reaction that sustains itself. That is, neutrons hit atoms, releasing more neutrons, which hit more atoms. The heat released is substantial and we usually use it to make steam to drive an electric generator.
Nuclear Fuel Used to Be Much Better
The original CP-1 reactor used natural uranium and so it took a lot of fuel since ordinary uranium is relatively stable. If that uranium is refined into higher concentrations of specific isotopes, a more powerful reaction is possible with less fuel.
There are three isotopes of uranium: U-238, U-235, and U-234. Uranium decays naturally, and highly active isotopes are now in short supply. Apparently, whatever process created the uranium on Earth, it all appeared at the same time (and has been decaying ever since) because no matter where you mine uranium today it is about 99.275% U-238.
U-235 is better for reactors but, today accounts for only 0.72% of what comes out of the ground. We enrich uranium artificially to have about 3% U-235 for use as fuel. Scientists think that when the Earth formed, U-235 was 30% of the crust’s uranium. That number keeps falling, of course, and 2 billion years ago, the proportion would be about 3.6% — just right for nuclear fuel.
When Mined Uranium Came up “Light”
In 1972 a French team mining uranium in Gabon noticed something strange. The ore didn’t have the expected 0.72% of U-235 but instead had 0.717%. That sounds like a tiny bit — and it is — but U-235 content is remarkably consistent all over the world (and on the moon, too). Further investigation confirmed that the missing U-235 was fissioned away in a natural nuclear reactor. In particular, all five fission products you’d expect were located. The amount of Xenon gas trapped in the reactor made it possible to calculate the operating cycle of the reactors.
Eventually, they would find 17 of these natural reactors. You might wonder why there are not even older natural reactors. After all, if 3.6% is a good fuel, wouldn’t 5% or 10% be even better?
The conditions had to be just right. A little over 2 billion years ago, there wasn’t much oxygen in the Earth’s atmosphere and this made it difficult for uranium to concentrate. So to get a natural reactor you need a concentration of uranium that has a high percentage of U-235. There also needs to be a suitable geometry of the material, a moderator like water, and no neutron-absorbing material in the area.
You can see in the figure that the uranium ore (3) is surrounded by porous sandstone (2) all on a layer of granite (4). The black zones (1) represent some of the natural fission zones.
Enough water would seep down through the sandstone and cause a reaction that would boil off the water within a half-hour. The reaction would then stop until sufficient water seeped in again creating a 3 hour total cycle time. This allowed the reactors to be very stable and appears to have allowed the reactors to run for about one million years.
There could be more reactors yet to be found or that operated and then did not survive the intervening 2 billion years for us to find. Conditions in Gabon were also just right to preserve the reactors.
If you are worried this could happen today, don’t fret. The U-235 content today is insufficient for this to happen again naturally. However, scientists have studied how waste products have been contained as it might lead to better ways for us to store nuclear waste we are creating.
As an interesting side note, there is a controversial theory that the moon was blown out of the Earth about 4.5 billion years ago by another natural reactor near the edge of the Earth’s mantle. While this hasn’t been disproven, as far as we can tell, most scientists seem to accept that the moon formed due to an impact on the early Earth.