Natural Nuclear Reactor

in May 1972 workers at a French reprocessing plant noticed something very suspicious about the uranium they were getting from one of their minds because the mine wasn't in France it was in Gabon West Africa a former French colony and they were mining the uranium there and then when they were looking at the concentration of uranium-235 remember that's the good stuff that's the fissionable things the fissile material when they were looking at that concentration they saw some missing and this is weird how do you suddenly not have the normal isotopic ratio how can there be less u-235 there then there is on the rest of the planet this is a place called Oak lobe and clearly something was going on so the scientists went down there and it's a fairly rich uranium mine there's some of the rocks you can even see that the yellowcake just being there in veins and some of these pockets actually were very dense with uranium so they took samples they took tests dug things out looked at stuff and what they found was absolutely amazing you see they found fission products they found that the ke products and elements and isotopes though it only ever exists if a nuclear reaction had taken place this was a natural nuclear reactor we all thought it was Fermi in 1942 who first figured out how to make fission in a lab or anywhere on the planet for heaven's sakes but two billion years ago this occurred why well remember about half lives today if I look at the isotopic abundance of u-235 its 0.7% and of course that means that the rest of the uranium the u-238 is 99.3 percent but the half-life of uranium 235 is 0.7 billion years and the half-life of uranium 238 is 4.5 billion years so if I go 700 million years in the past this doesn't become point seven percent becomes one point four percent if I go another half-life that becomes closer to three percent this one's going down a little so it's not exact but approximately two billion years ago this was 3% and 97% 3% is the number you need to be able to have fission these days we have reprocessing and we have enrichment plants that up this percentage to three percent so we can run it in a nuclear reactor if you just went 2 billion years back in time all of the uranium you could dig up around the planet was 3% u-235 but of course it takes more than just having the right isotopic abundance to be able to make fission what you also need is a moderator you need something that slows down the neutrons makes them thorough in which case they can then interact with another u-235 and cause a fission release more neutrons a moderator has to slow those neutrons down then again you can have this sustained chain reaction so how would you get a moderator well remember our typical moderator is water so maybe if I had just the right geology I could have a natural nuclear reactor so how could you have that particular geometry of the geology maybe it could look something like this this is a rough cross-section map of the Oh coal Oh mine in Gabon West Africa and this band you see here the number three that's an underground river now don't think an underground river is like some amusement park ride where you're on a boat and you're going through a tunnel on water underground rivers don't look like that underground rivers are collections of gravel of permeable rock sand things water can flow through so if I have a band of things where water can throw flow through and some like impermeable bedrock so that water would collect up here and then flow this way and then these black areas these are actually the uranium deposits we are set up to have a reactor step 1 rains a lot water pools in the water comes through we have a moderator now if you have a chain nuclear reaction you have some energy we're not talking about atom bombs this isn't ninety percent u-235 right but what's gonna happen is very interesting it's gonna get warm and that water is gonna boil that's gonna boil away but remember one of the inherent safety features of nuclear reactors if the water boils away no moderator and the reaction stops by carefully looking at the isotopes that were left and figuring out what the operation pattern must have been it seemed like maybe it could operate for 30 minutes who'd be off for two and a half hours or so maybe come back on for 30 minutes if the water rushed back into that area and continued this from millions of years whatever there was a really big rain and the water would come down just right it doesn't make mushroom clouds it doesn't you know blow up over the ground it just boils the water away sort of like a Old Faithful builds up enough heat when the water comes back in the moderator comes back in this graph shows four but there are actually sixteen pockets that did this and then as scientists looked further in other regions in Africa this wasn't the only place that had a natural nuclear reactor what can we learn about this well we can learn a couple things the first is that sort of the hubris of mankind that whoa we've invented everything well nature always has ways of surprising us this is interesting the nature had a nuclear reactor but the other is to look at the waste products you see in a nuclear reactor everything is designed to keep those fission products in place first you take an oxide fuel that's a ceramic so it won't melt then you put that into a fuel pellet the pellet isn't a fuel pin the pin is in a fuel rod the rods and a reactor vessel the reactor vessels in the containment building these fission products they're in an underground river there's washed downstream so when scientists go to say where are these unique isotopes where are these fission products the end of the decay chain that's how we know their nuclear reactor really happened there these elements should not exist in nature finding them and finding their distribution away from the spots where they were created gives us an idea about waste migration over geologic timescales and the answer is surprising they didn't go very far so when people say how could you ever imagine storing nuclear waste and thinking that it would stay there for millions of years Oh Club West Africa that's what you need to know about natural nuclear reactors you [Music] you

Channel: Illinois EnergyProf

Views: 188,080

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Id: pMjXAAxgR-M

Length: 9min 26sec (566 seconds)

Published: Tue May 14 2019