Earth's Two-Billion-Year-Old Nuclear Reactor
Earth's Two-Billion-Year-Old Nuclear Reactor
In this fun episode of SciShow, we’ll take you to a uranium deposit in Africa where, eons ago, a unique set of conditions came together to form the world’s only known natural nuclear reactor! Sound crazy? Join us! No radiation suit required! Hosted by: Hank Green.
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Content
11.74 -> In 1972, French scientists were testing some uranium taken from the Oklo mine in Gabon, in Central Africa,
17.64 -> where they had noticed something odd. They had a lot less usable uranium than they thought they did.
22.48 -> Enough uranium to make six nuclear bombs was just... missing.
26.02 -> Normally the uranium found in ore comes in three different varieties, or "isotopes,"
30.14 -> each with a different number of neutrons in its nuclei.
33.32 -> Since their atomic make-ups are slightly different, they all react in different ways.
36.6 -> But when you're looking for the stuff that releases enough energy to light up a city
39.66 -> or vaporize an island when you split it,
41.96 -> the isotope you're looking for is Uranium-235.
44.84 -> Now, Uranium ore should contain the same ratios of different isotopes
49.18 -> no matter where it's found in the world.
50.84 -> So the French scientists knew that, in the ore they were studying,
53.68 -> they were supposed to find a concentration of Uranium-235 that was exactly 0.72%.
60.2 -> But it turns out they only had a concentration of 0.717%.
64.36 -> Now it might not sound like a lot,
65.48 -> but spread out over the giant mine at Oklo
67.7 -> it looked like 200 kilograms of Uranium-235 had disappeared.
71.62 -> After a few weeks of frantic investigation,
73.7 -> the scientists realized that they were NEVER going to find the missing Uranium
77.12 -> because it had been gone for two billion years.
80.16 -> What they had discovered was the remains of an ancient natural nuclear reactor.
85.3 -> Some physicists had theorized that certain kinds of nuclear reactors could exist naturally,
90.52 -> but only under very specific conditions.
92.76 -> And it turns out that the Uranium deposits at the Oklo mine and the area around it
96.74 -> fulfilled ALL of those conditions.
99.14 -> Basically, a natural nuclear reactor needs the same ingredients that modern nuclear power plants do.
103.64 -> You need an atom that you can split pretty quickly and easily,
106.48 -> you need neutrons to do the splitting,
108.44 -> and you need a way to control that reaction, so once the atoms start breaking apart
112.78 -> and releasing face-melting amounts of energy,
114.58 -> you don't have a nuclear Armageddon on your hands.
117.36 -> When the first ingredient, an atom of Uranium-235, is struck by the second ingredient, a stray neutron,
122.8 -> the atom becomes so unstable that it splits,
125.42 -> releasing a ton of energy, plus a few more neutrons that fly off
128.94 -> and split other Uranium atoms, creating a chain reaction.
131.78 -> And way back, a couple of billion years ago, Oklo had both of those ingredients.
136.38 -> Back then, natural concentrations of Uranium-235 pretty much everywhere on the planet
140.82 -> were at a healthy 3%, which is about as much as nuclear power plants use in their fuel rods today.
145.64 -> And in addition to all that potential fuel, Oklo also has lots of neutrons flying around
150.16 -> just because, over time, Uranium-235 naturally decays into element, an isotope of Thorium,
155.62 -> and in the process, it releases a neutron.
157.58 -> Now, in a natural, uncontrolled reaction, all of those neutrons being released
161.04 -> would be moving WAY too fast to split an atom.
163.84 -> Instead of hitting another atom, they'd mostly just rocket away without hitting anything,
167.7 -> and the reaction would simply stop.
169.3 -> But this didn't happen at Oklo because the deposit also had the third major ingredient:
173.52 -> a way to control the reaction.
175.52 -> The Oklo deposit was full of water. Groundwater, to be precise,
179.26 -> which continuously flowed into the site, slowing the neutrons down enough
183.46 -> that they had time to hit other atoms of Uranium-235 and keep the chain reaction going.
188.06 -> Now you'd think with all these atoms getting split and energy being released,
191.52 -> things would spin out of control pretty fast.
193.52 -> But it turns out that the groundwater prevented that from happening as well.
196.48 -> As more and more Uranium split, the energy being released made the water hotter.
201.4 -> After about half an hour of that, the water would get so hot that it would start to boil off.
205.12 -> And without the groundwater to slow them down,
206.8 -> the neutrons would start moving too fast to hit any more Uranium
209.54 -> which would quickly stop the reaction.
211.34 -> But eventually, the water would come back and slow down the neutrons enough
214.5 -> that the chain reaction would start over.
216.42 -> And on and on this cycle went, likely for hundreds of thousands of years.
220.94 -> All told, the deposits in the Oklo area
222.6 -> probably had an average ongoing power output of about a hundred kilowatts,
226.56 -> which is enough to power about a hundred washing machines
228.9 -> FOR EONS.
230.24 -> None of the energy was ever put to any use;
232.06 -> eventually, the reaction used up so much of the U-235 in the deposit
235.52 -> that the concentration got too low, and the reaction stopped.
238.62 -> So, the energy produced by Oklo's natural nuclear reactor quietly dissipated,
242.54 -> leaving only that missing Uranium and a few byproducts as evidence that it ever existed.
247.54 -> It's pretty cool that nature actually figured out how to build a nuclear reactor before we did!
251.26 -> Thank you for watching this SciShow Dose!
253.18 -> If you have any questions for us, we're on Facebook and Twitter and Tumblr and down in the comments below,
257.5 -> and if you wanna keep getting smarter with us,
259.46 -> you can go to youtube.com/SciShow and subscribe.
Source: https://www.youtube.com/watch?v=yS53AA_WaUk