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Ancient Nukes: 1.9 Billion-Year-Old Nuclear Reactors · PlanetGeo: The Geology Podcast

• 0:03
  SPEAKER_01: Welcome to Planet Geo, the podcast where we talk about our amazing planet, how it works, and why it matters to you.
• 0:12
  SPEAKER_01: Chris, how are we doing today?
• 0:14
  SPEAKER_00: Doing awesome.
• 0:15
  SPEAKER_00: How are you doing, Jesse?
• 0:16
  SPEAKER_01: Yeah, I'm great.
• 0:17
  SPEAKER_01: I'm great.
• 0:17
  SPEAKER_01: I'm great.
• 0:18
  SPEAKER_01: I am pretty excited about this episode.
• 0:20
  SPEAKER_00: Yeah, you should be.
• 0:21
  SPEAKER_00: This is your idea.
• 0:23
  SPEAKER_01: Um, well, before we get into it, let's do some introductions.
• 0:27
  SPEAKER_01: Jenny just brought me a beer.
• 0:30
  SPEAKER_01: Look at her.
• 0:31
  SPEAKER_01: Wow, look at that.
• 0:32
  SPEAKER_01: Way to go, Jenny.
• 0:35
  SPEAKER_00: Maybe you don't want to cut this out.
• 0:36
  SPEAKER_00: I don't know.
• 0:37
  SPEAKER_01: Yeah, maybe.
• 0:37
  SPEAKER_01: Um, can you deliver get one delivered to me too?
• 0:40
  SPEAKER_01: That'd be great.
• 0:42
  SPEAKER_00: Jesse wants a beer too, please.
• 0:45
  SPEAKER_00: Okay.
• 0:45
  SPEAKER_00: Yeah.
• 0:46
  SPEAKER_00: She just delivered me a gigantic ass beer.
• 0:48
  SPEAKER_00: So there you go.
• 0:49
  SPEAKER_00: Good.
• 0:49
  SPEAKER_00: That's great for podcast recording.
• 0:51
  SPEAKER_00: It's a it's king size, yeah.
• 0:53
  SPEAKER_00: All right.
• 0:53
  SPEAKER_00: So yeah.
• 0:54
  SPEAKER_00: Well, you should be excited, Jesse, because this is your idea right from the get-go.
• 0:58
  SPEAKER_01: Well, you're gonna have to uh keep me concise and prevent me from rambling on this one, I think.
• 1:03
  SPEAKER_00: So I'll I'll try, I'll do my best.
• 1:05
  SPEAKER_01: All right, all right.
• 1:06
  SPEAKER_01: Well, before we get into it, real quick introductions.
• 1:08
  SPEAKER_01: You are Chris Bullheis, high school science teacher in Michigan, highly credentialed high school science teacher in Michigan, and actually my former teacher.
• 1:17
  SPEAKER_00: Yes, and you are Jesse Rymank.
• 1:19
  SPEAKER_00: Doctor, I'm sorry, I gotta say this.
• 1:20
  SPEAKER_00: I'm gonna start it over.
• 1:21
  SPEAKER_00: And you are Dr.
• 1:22
  SPEAKER_00: Jesse Rymanck.
• 1:23
  SPEAKER_01: There we go.
• 1:23
  SPEAKER_00: As you just said, one of my former students, and uh, you are now professor of geoscience at Penn State University.
• 1:30
  SPEAKER_01: That's right.
• 1:31
  SPEAKER_01: And so, what are we talking about today?
• 1:33
  SPEAKER_00: Well, all right, today took some convincing uh on your part.
• 1:40
  SPEAKER_00: Like, what are you talking about?
• 1:43
  SPEAKER_00: So we are gonna talk about well, the we kind of titled this Ancient Nukes.
• 1:48
  SPEAKER_00: Yeah, um, we're gonna talk about natural nuclear reactors.
• 1:52
  SPEAKER_00: That's right.
• 1:53
  SPEAKER_00: And when you first like brought, I'm like, what?
• 1:56
  SPEAKER_00: Uh you're gonna have to do you're gonna have to do some work on this.
• 1:59
  SPEAKER_00: And but but you know, like you sold it really when you said this is my coolest factoid in geology.
• 2:08
  SPEAKER_01: Yeah, totally.
• 2:09
  SPEAKER_01: I mean, this is this is the single like coolest thing that I know about in the geosciences.
• 2:13
  SPEAKER_01: When I learned about this stuff, I was just totally engrossed in it.
• 2:17
  SPEAKER_01: It's so cool, and I think it's very cool.
• 2:20
  SPEAKER_01: A, it also highlights a couple of very important links with the geosciences.
• 2:25
  SPEAKER_01: And so let's go through a couple of those real quick.
• 2:27
  SPEAKER_01: Like, why why should people care about these ancient newts?
• 2:30
  SPEAKER_01: Let's let's run through them real quick.
• 2:31
  SPEAKER_00: Yeah, and then right after that, Jesse, I want you to just give us a summary about where we're headed with this episode.
• 2:37
  SPEAKER_00: What are we gonna do?
• 2:38
  SPEAKER_01: Okay.
• 2:38
  SPEAKER_00: So, first and foremost, geology and nuclear physics have a very intimate relationship.
• 2:43
  SPEAKER_00: You know, we did an episode on geologic time and radiometric dating.
• 2:47
  SPEAKER_00: Um, and the backbone of this research was done on uranium nuclear physics and research into nuclear weapons dating back to World War II.
• 2:57
  SPEAKER_01: Yeah, that's right.
• 2:58
  SPEAKER_01: It's suit, you know, basically all of the geologic timeline is constrained by radiometric dating, which has its basis in nuclear physics.
• 3:06
  SPEAKER_01: The other thing is that geoscience and national security are also intimately related.
• 3:11
  SPEAKER_01: And what I mean by that is that the instruments that we use in geology and geochemistry, the instruments that are in my lab that I'm working to improve upon the designs of, are used for what are called nuclear forensics.
• 3:25
  SPEAKER_01: And this is basically people looking for other countries enriching uranium or creating atomic weapons, basically.
• 3:33
  SPEAKER_00: So and that's basically how this, what we're gonna talk about, the site where this happened, right?
• 3:38
  SPEAKER_00: That's how it was found.
• 3:38
  SPEAKER_00: Is that right?
• 3:40
  SPEAKER_01: Exactly, exactly.
• 3:41
  SPEAKER_01: And so there are a lot of people who get PhDs in geoscience studying rocks who end up going to work in national security labs around the country, at least in the US and also in the EU, uh, and in many other countries.
• 3:54
  SPEAKER_01: So it's really there's an intimate link there with both techniques and expertise.
• 3:59
  SPEAKER_00: Yeah, and then I I I found something else that was really interesting about how these the way these concentrated deposits form is related to early Earth's atmosphere.
• 4:11
  SPEAKER_00: So studying these ancient nuclear reactors help us understand Earth's early history, and so it's intertwined there too, and how these natural deposits occurred.
• 4:21
  SPEAKER_00: So, you know what?
• 4:22
  SPEAKER_00: You had to sell me on this, but I it took I'm I'm all in.
• 4:26
  SPEAKER_01: It took it took some convincing, but you're on board now.
• 4:28
  SPEAKER_01: Exactly.
• 4:29
  SPEAKER_00: It is this is one of the interesting things about doing this podcast, Jesse, is the discussions that you and I have before and after, and you know, um, we it's it's interesting how we really need to sell each other on our ideas on how to do this podcast and so on.
• 4:46
  SPEAKER_00: It's an interesting uh dynamic that I didn't really expect.
• 4:49
  SPEAKER_01: Yeah, yeah, it's fun.
• 4:50
  SPEAKER_01: And sometimes we get into arguments about it, and other times and then we get over it.
• 4:54
  SPEAKER_01: So, all right, so where are we going with this episode?
• 4:56
  SPEAKER_01: Uh, this is we got to cover some basics that are a little bit complicated.
• 5:00
  SPEAKER_01: So we've spent a lot of time trying to work out the perfect analogies for this podcast, but where we're going is first we're gonna talk about nuclear chain reactions, and we're gonna really kind of hone in on the basic principles of nuclear reactors.
• 5:14
  SPEAKER_01: Then we're gonna step back and kind of tell the story of the discovery and study of these ancient natural nuclear reactors that occurred on Earth.
• 5:22
  SPEAKER_01: We're gonna end by kind of linking in how uranium deposits form on the ancient earth and the modern earth, and just highlight a couple takeaway points at the end uh that we can take away from understanding these ancient uh nukes, basically.
• 5:36
  SPEAKER_00: Yeah, because it is such a cool story, and uh you know we're gonna we're gonna end up talking about how this is really similar to old faithful.
• 5:44
  SPEAKER_01: Yeah, exactly.
• 5:45
  SPEAKER_00: That's such a I think that was the point that sold me.
• 5:48
  SPEAKER_00: I'm like, really?
• 5:48
  SPEAKER_00: What?
• 5:49
  SPEAKER_01: If I ever need if I ever need to sell Crisple S on something, say Yellowstone, bring Yellowstone into it, and you know you're gonna you're gonna win.
• 5:55
  SPEAKER_01: Yeah, that's true.
• 5:56
  SPEAKER_00: That's true, that's true.
• 5:57
  SPEAKER_01: Hey, so let's go.
• 5:58
  SPEAKER_01: Yeah, so let's get into it.
• 5:59
  SPEAKER_01: So we need to explain nuclear reaction.
• 6:02
  SPEAKER_01: Let's get us all on the same page of nuclear reactions and specifically the different isotopes of uranium and 235 uranium is the one we're gonna kind of focus in here.
• 6:10
  SPEAKER_00: Uh basically, we're gonna talk about the two isotopes of uranium that are important here, uranium-238 and uranium-235.
• 6:18
  SPEAKER_00: Um, and it's the uranium-235 that is important because U235 is the fissionable uranium, and it has a shorter half-life than uranium-238 does.
• 6:27
  SPEAKER_00: So basically, what happens to uranium-235 is that isotope, if it gets hit with a neutron moving at the proper speed, it absorbs that neutron, becomes very unstable, and breaks apart.
• 6:42
  SPEAKER_00: And that's what we call fission is the breaking apart of the nucleus of an atom.
• 6:46
  SPEAKER_00: And so that uranium flies apart into two now smaller isotopes that we don't really, I think, need to get into, right?
• 6:53
  SPEAKER_00: Yeah.
• 6:53
  SPEAKER_00: Jesse.
• 6:54
  SPEAKER_00: Okay.
• 6:55
  SPEAKER_00: So anyway, but the important part about it is that you have three more neutrons, or you have the original neutron plus two more neutrons that come off at nearly the speed of light, and they will also then slam into uranium if there's enough of it present.
• 7:11
  SPEAKER_00: And the process repeats itself.
• 7:13
  SPEAKER_00: So basically, you have what we call this chain reaction, where you know, one neutron leads to three, three neutrons lead to nine, nine leads to 27, and on uh, you know, this so this kind of like chain spreading uh reaction of splitting uranium-235 atoms is going on.
• 7:31
  SPEAKER_01: And so, you know, there's an analogy that before we get to the analogy, you mentioned on that there's enough 235 uranium, and it's dependent upon this ratio or this relationship between 235 uranium and 238 uranium.
• 7:42
  SPEAKER_01: So can you dive into that a little bit?
• 7:44
  SPEAKER_00: Yeah.
• 7:45
  SPEAKER_00: So basically, in order to have uh uh what we call the critical mass, a fissionable amount of uranium, you have to have about three and a half percent uranium-235 in relation to uranium-238.
• 7:58
  SPEAKER_01: And that's what we call enriched uranium.
• 8:00
  SPEAKER_01: So when you hear, you know, like Iran is enriching uranium or something like that, that's what they're talking about.
• 8:06
  SPEAKER_01: Because the background amount of uranium, the natural amount of uranium, is only 0.7% 235 uranium, right?
• 8:14
  SPEAKER_00: Right, it is.
• 8:14
  SPEAKER_00: And that's because you know, uranium-235 has a shorter half-life.
• 8:18
  SPEAKER_00: So with the age of the earth, the naturally occurring uranium that we have on earth, we're at 0.7% now.
• 8:26
  SPEAKER_00: But when this happened, uh and we'll talk about the timeline later on, there was enough of it there.
• 8:33
  SPEAKER_00: There was three and a half percent of the uranium-235.
• 8:37
  SPEAKER_00: So, anyway, the chain reaction, you came up with a really good analogy, and I'll just kind of run it down real quick.
• 8:42
  SPEAKER_00: Think about a floor, a room in your house, and the floor is covered with mouse traps, just all right next to each other, and these mouse traps all have ping pong balls on them.
• 8:51
  SPEAKER_00: So if you stand back from this floor covered with these mouse traps and ping pong balls, and you throw a ping pong ball in the middle of it, you set one of them off, and this ball goes flying and it sets off other traps and other traps until you know all these traps are going off in kind of this uncontrolled way.
• 9:12
  SPEAKER_00: That's the chain reaction that uranium-235 has.
• 9:16
  SPEAKER_00: That's what these neutrons do, where the the one leads to three, the three leads to nine, and so on and so on and so on, if there's enough uranium present.
• 9:25
  SPEAKER_01: Right.
• 9:25
  SPEAKER_01: And the important thing on that analogy is when you throw the first ping pong ball in the air, it triggers a trap, and that so another one gets sent in the air, but the first ping pong ball also gets sent in the air.
• 9:34
  SPEAKER_01: And so you two becomes four, et cetera, et cetera, down the road.
• 9:37
  SPEAKER_01: Um, and so this first event, the first ping pong ball being thrown onto this floor, is spontaneous fission.
• 9:44
  SPEAKER_01: And spontaneous fission happens in the background, it's a pretty rare event, but it does happen.
• 9:48
  SPEAKER_01: Uh, and so there are neutrons flying around in the background that can start these chain reactions.
• 9:53
  SPEAKER_01: We just need to have a situation where you have the floor covered with mouse traps with ping pong balls on them to get the chain reaction going and to sustain it.
• 10:02
  SPEAKER_01: So the other thing that a chain reaction requires is what's called a neutron moderator, which is kind of a bad name.
• 10:10
  SPEAKER_01: Yeah, I agree.
• 10:12
  SPEAKER_01: I don't like it.
• 10:13
  SPEAKER_01: It doesn't really, it doesn't moderate anything.
• 10:16
  SPEAKER_01: I mean, it moderates the neutrons, but it helps the reaction go forward.
• 10:20
  SPEAKER_01: So it actually increases the reaction rate.
• 10:22
  SPEAKER_01: And what this is is basically you need to have some element around that slows down the neutrons because 235 uranium, it will absorb slow neutrons, but it will not absorb fast neutrons.
• 10:34
  SPEAKER_01: And so you need to slow these neutrons down, you need to slow the ping pong balls down so that they have the potential to spring more mouse traps.
• 10:42
  SPEAKER_01: So you gotta have this moderator around, uh, and it can be water, it can be graphite, it can be molten salt, it can be a lot of different things, but you have to have a moderator.
• 10:51
  SPEAKER_01: And the moderator is again moderating the neutron speed, it's actually increasing the reaction rate.
• 10:57
  SPEAKER_01: Okay.
• 10:58
  SPEAKER_01: So we'll come back to that later in the old faithful analogy.
• 11:01
  SPEAKER_00: So, Jesse, what about the uranium-238?
• 11:03
  SPEAKER_00: Yeah, does it not split?
• 11:05
  SPEAKER_00: Does it not undergo fission?
• 11:07
  SPEAKER_00: Right.
• 11:07
  SPEAKER_01: So 238, it will undergo fission, but just not nearly as much as 235 uranium.
• 11:11
  SPEAKER_01: It's much less likely to happen.
• 11:13
  SPEAKER_01: So it kind of poisons the reaction.
• 11:15
  SPEAKER_01: So this is why we need to have quote unquote enriched uranium where there's 3.5% 235 uranium around to have a sustainable reaction because 238 will poison the reaction.
• 11:27
  SPEAKER_01: If there's too much 238 uranium around, it'll poison right.
• 11:30
  SPEAKER_00: So, what happens, Jesse, when uranium-235 does split?
• 11:35
  SPEAKER_00: What is what's the end result of this?
• 11:37
  SPEAKER_01: So 235 uranium, that atom will break apart.
• 11:41
  SPEAKER_01: The nucleus breaks apart, releases a ton of energy, it releases a bunch of neutrons, and it also forms little fragments.
• 11:48
  SPEAKER_01: So you'll form things like barium or xenon or other elements that are smaller, they have smaller nuclei, and they're just fragmented 235 uranium.
• 11:57
  SPEAKER_01: So it releases a bunch of neutrons.
• 11:59
  SPEAKER_01: Those neutrons go around and hit other 235 uranium, and then this chain reaction progresses.
• 12:05
  SPEAKER_00: And then you also accompanying this split is a conversion of a little bit of mass into a lot of bit of energy.
• 12:13
  SPEAKER_01: That's right.
• 12:14
  SPEAKER_01: That's right.
• 12:14
  SPEAKER_01: And so going back to the mouse trap analogy, if you imagine 238 uranium as this poison to the reaction, 238 uranium is if you have a bunch of mouse traps on the floor, but they don't have ping pong balls resting on them.
• 12:27
  SPEAKER_01: So a ping pong ball comes in and triggers them, and it doesn't release two ping pong balls, it only kicks off one.
• 12:33
  SPEAKER_01: That will eventually poison this reaction and slow it down and eventually stop the chain reaction if there's enough in it.
• 12:39
  SPEAKER_00: So the uranium-238 that is removing neutrons.
• 12:43
  SPEAKER_00: Exactly.
• 12:44
  SPEAKER_00: Slowing it down.
• 12:44
  SPEAKER_00: Okay, I gotcha.
• 12:45
  SPEAKER_01: All right, cool.
• 12:46
  SPEAKER_01: Exactly.
• 12:47
  SPEAKER_01: All right.
• 12:47
  SPEAKER_01: Well, hey, let's move on, Jesse.
• 12:49
  SPEAKER_01: All right, so let's move into the story of the discovery of these ancient nukes.
• 12:54
  SPEAKER_01: And this is a really kind of interesting one, and it highlights this integration between geoscience and uh nuclear forensics and nuclear physics because in the 1970s there were some French scientists that were working for one of the atomic energy agencies, and they're measuring the isotope composition or the ratio of 235 uranium to 238 uranium from a bunch of different samples.
• 13:16
  SPEAKER_01: They were measuring things to look for other countries enriching uranium.
• 13:20
  SPEAKER_00: So they were so wait a minute, they were looking at uranium to see if somebody was taking uranium to enrich it to 3.5%.
• 13:28
  SPEAKER_00: Is that what you're saying?
• 13:29
  SPEAKER_01: Exactly.
• 13:29
  SPEAKER_01: Because if some country is enriching uranium, they're increasing 235, what happens is you also generate depleted uranium, which has 235 taken out of it.
• 13:39
  SPEAKER_01: So if you look at uranium around in the water or in some mining tailings pile or something, and you look at the isotope composition and you're missing 235 uranium, you can say, ah, somebody's taken that 235 uranium.
• 13:51
  SPEAKER_01: They might be making a bomb.
• 13:53
  SPEAKER_01: Does this happen often?
• 13:54
  SPEAKER_01: Do scientists are this happens all the time.
• 13:56
  SPEAKER_01: Yep.
• 13:57
  SPEAKER_01: A lot of these national labs, a lot of these PhD geochemists who go work for national labs are doing exactly this type of measurement.
• 14:03
  SPEAKER_00: Wow.
• 14:03
  SPEAKER_01: It's very interesting.
• 14:04
  SPEAKER_00: I think that's that's really interesting.
• 14:06
  SPEAKER_00: Um, do we go to other countries?
• 14:08
  SPEAKER_00: Do we send our scientists to other countries to do this?
• 14:12
  SPEAKER_00: Good question.
• 14:12
  SPEAKER_01: I don't actually know how the sampling is done.
• 14:15
  SPEAKER_01: A lot of the people that are doing the measurements are just in the lab doing the measurements.
• 14:20
  SPEAKER_01: It's all security compartments.
• 14:22
  SPEAKER_01: So people making the measurements don't know where the samples come from.
• 14:25
  SPEAKER_01: They're kind of anonymously labeled.
• 14:27
  SPEAKER_01: And so, you know, only the high, high up, secure, ultra secure people know the full lifetime of that or the full chain of that sample and can relate it to the location.
• 14:37
  SPEAKER_01: But this was done in Gabon.
• 14:40
  SPEAKER_01: Yes, Oaklose.
• 14:41
  SPEAKER_01: Exactly.
• 14:41
  SPEAKER_01: So in the French labs, people were uh French scientists were measuring uranium, isotope composition from different samples around the world.
• 14:49
  SPEAKER_01: They were actually measuring their standards, though, which are supposed to have 0.7% of uranium be 235 uranium.
• 14:55
  SPEAKER_01: Instead, what they measured, so we're gonna throw some numbers in here just to highlight the precision of these analyses.
• 15:00
  SPEAKER_01: So what they're expecting to see is 0.719% of the uranium should be 235 uranium.
• 15:08
  SPEAKER_01: So 0.719%, that's what we're expecting.
• 15:11
  SPEAKER_01: Instead, what they found was 0.716% was 235 uranium.
• 15:17
  SPEAKER_00: That's a small difference.
• 15:18
  SPEAKER_00: Extremely small difference.
• 15:20
  SPEAKER_01: It's a very small difference, but it's significant.
• 15:23
  SPEAKER_01: It highlights how well we can and need to make these measurements in order to do the things that nuclear security relies upon.
• 15:30
  SPEAKER_00: I want to make a point here, Jesse.
• 15:32
  SPEAKER_00: What you're saying is that all uranium naturally occurring uranium on the planet should have 0.72% U235.
• 15:42
  SPEAKER_01: That's right.
• 15:43
  SPEAKER_01: That's right.
• 15:44
  SPEAKER_00: And that's what we universally find.
• 15:46
  SPEAKER_01: For the most part, yes, that is absolutely true.
• 15:48
  SPEAKER_01: There is there is a little bit of deviation from that naturally, but uh for the most part, yes.
• 15:52
  SPEAKER_00: That is an astounding fact because we did an episode on geologic time and geochronology and the age of the earth and so on.
• 15:59
  SPEAKER_00: And wow, what a that's a very compelling line of evidence, the ratio of U235 and U238 that have different half-lives and what we get on this planet.
• 16:10
  SPEAKER_01: Um very compelling.
• 16:12
  SPEAKER_01: Yeah, it's super interesting.
• 16:14
  SPEAKER_01: So it was shocking for these scientists to say, oh, we have this natural, what should be a standard, which we should know the composition, and it's wrong.
• 16:20
  SPEAKER_01: It's actually 0.716.
• 16:22
  SPEAKER_01: They thought, wow, somebody has taken a lot of 235 uranium from this place that we didn't know about it.
• 16:29
  SPEAKER_01: So they had the wrong value.
• 16:30
  SPEAKER_01: There's missing 235 uranium, and they're thinking, who took it?
• 16:33
  SPEAKER_01: Where did it go?
• 16:34
  SPEAKER_01: Again, this is typical of depleted uranium, where somebody has extracted some of the 235 out of it to make a bomb or to make a nuclear reactor somewhere.
• 16:44
  SPEAKER_00: That is very interesting.
• 16:45
  SPEAKER_00: So, just for perspective, about 4,500 kilograms.
• 16:51
  SPEAKER_00: In other words, five tons of uranium-235 is gone.
• 16:58
  SPEAKER_01: It's um so much, so much is gone.
• 17:02
  SPEAKER_00: It is almost 10,000 pounds.
• 17:04
  SPEAKER_00: And for reference, the little boy bomb, one of it was the first atomic weapon ever used.
• 17:09
  SPEAKER_00: Yeah, the one dropped on Hiroshima, yeah, had 64 kilograms of U235 in it, not all of which was converted into energy.
• 17:18
  SPEAKER_00: You know, just a small part of that was actually converted into energy.
• 17:21
  SPEAKER_00: So that's a lot of energy in these natural nuclear reactors.
• 17:27
  SPEAKER_00: Um, and here's a factoid for you.
• 17:30
  SPEAKER_00: If you convert the mass of a paper clip, about two grams, if you convert that into energy, then that is the equivalent of striking a match to about 700,000 gallons of gasoline.
• 17:45
  SPEAKER_01: That's amazing.
• 17:46
  SPEAKER_01: Two grams.
• 17:47
  SPEAKER_01: You know, it's like it is, and so the E equals M C squared equation is just so powerful in this thing.
• 17:52
  SPEAKER_01: So they were missing around about 70 bombs worth of 235 uranium from this area where this uranium came from, which is in Gabon in Africa.
• 18:04
  SPEAKER_00: So going back to the whole uranium-235 to uranium-238, and why when this happened, there was three and a half percent uranium-235 and 96.5% of uranium-238.
• 18:19
  SPEAKER_00: Why was it that way?
• 18:20
  SPEAKER_00: Well, because they have different half-lives.
• 18:23
  SPEAKER_01: Sorry, because let me clarify that.
• 18:24
  SPEAKER_01: You mean why, why, when the nuclear reactor, when this natural nuclear reactor we are talking about, occurred.
• 18:29
  SPEAKER_00: Yes, that's what I'm talking about.
• 18:30
  SPEAKER_00: Yep, yeah, exactly.
• 18:31
  SPEAKER_00: Okay, and if you think about back to with the geochronology episode that we did uh a couple weeks ago, uh, we used a shoebox analogy, and we can use that here too.
• 18:41
  SPEAKER_00: Let's say if you put 5,000 pennies and 5,000 dice in the shoebox.
• 18:48
  SPEAKER_00: Okay, it'd be if it's a big shoebox, but anyway, you put that in there and you shake it up, and you when you take the top off, you remove all the tail sides up, right?
• 18:57
  SPEAKER_00: That's the uranium 235.
• 18:59
  SPEAKER_00: It has a shorter half-life, so it decays faster.
• 19:02
  SPEAKER_00: So you would remove half of those, but we're only gonna remove the dice that are number one side up.
• 19:10
  SPEAKER_00: Okay, you rolled a one, so it's a one out of six probability then that you're gonna take that out.
• 19:14
  SPEAKER_00: So it would take many more, you know, shakes of the box to remove the dice that are one side up than it would take to remove half of the pennies.
• 19:25
  SPEAKER_01: That's right.
• 19:25
  SPEAKER_01: And so I love this shoebox analogy.
• 19:27
  SPEAKER_01: I mean, you came up with this for the I think you use this in class, but this is a gem of an analogy.
• 19:32
  SPEAKER_01: So, what we're talking about here for this natural nuclear reactor is we're doing that in reverse.
• 19:37
  SPEAKER_01: So we're basically we're using that shoebox analogy and we're working backward with that, which means that if we start from 0.7% of the uranium on Earth today is 235 uranium.
• 19:49
  SPEAKER_01: If we go backward in time, we know that 235 uranium decays faster than 238.
• 19:54
  SPEAKER_01: So if we go back in time, 235 uranium is going to be more of a percentage of total uranium back in time.
• 20:03
  SPEAKER_01: And when we get to about 1.5 to 1.7 billion years ago, this is where the percentage of 235 uranium on Earth is around about 3, 3.5%, which is our kind of Goldilocks percentage that we're looking for for sustainable fission reactors.
• 20:19
  SPEAKER_00: Okay, Jesse, let's step back and think about how uranium deposits form.
• 20:23
  SPEAKER_00: How does this concentrate?
• 20:26
  SPEAKER_00: Why did it concentrate 1.7 billion years ago?
• 20:30
  SPEAKER_00: Um, it can't anymore do this because so much of it has decayed down.
• 20:35
  SPEAKER_00: We're we're not at the critical mass, and it didn't happen prior to 1.7 billion years ago, at least we don't think it did.
• 20:42
  SPEAKER_00: So walk us through that.
• 20:44
  SPEAKER_00: How did uranium enrich?
• 20:46
  SPEAKER_01: First, let me add a caveat in here in that we still don't know if more ancient nuclear reactors occurred out there in the planet.
• 20:54
  SPEAKER_01: There could have been other ones, we just haven't detected them.
• 20:57
  SPEAKER_01: So we need two things to have a nuclear reaction.
• 21:00
  SPEAKER_01: We need to have a lot of 235 in the system.
• 21:03
  SPEAKER_01: So we need to have more than 0.7%.
• 21:05
  SPEAKER_01: We need to have about 3% of the uranium be 235 uranium.
• 21:09
  SPEAKER_01: We also need to have a lot of uranium, so we need a way to concentrate uranium into an area that can sustain this nuclear reaction.
• 21:19
  SPEAKER_01: And we do this in nuclear power plants today.
• 21:21
  SPEAKER_01: We just take a bunch of uranium, chemically separate it, put it all together, enrich the 235 and put it in a nuclear power plant.
• 21:27
  SPEAKER_01: That is what is generating power.
• 21:29
  SPEAKER_01: But back in time, we needed a way to concentrate uranium naturally in Earth.
• 21:34
  SPEAKER_01: And this is intimately linked to oxygen in the atmosphere.
• 21:37
  SPEAKER_01: And the details here we're not going to get into too much, but suffice to say that if there's oxygen in the atmosphere, uranium can be moved in groundwater.
• 21:46
  SPEAKER_01: It's soluble, it's dissolved in groundwater.
• 21:48
  SPEAKER_00: So, what you're saying is we need oxygen in the atmosphere in order for uranium to be dissolvable in water.
• 21:56
  SPEAKER_01: Exactly, exactly.
• 21:57
  SPEAKER_01: And so if we go back in time, we To about 2.4 billion years ago in Earth history, and there's no oxygen in the atmosphere prior to that, or at least very, very, very little oxygen, not enough to have uranium dissolved in water, moves uranium around, and has the ability to concentrate uranium in certain locations.
• 22:15
  SPEAKER_01: So we have this Goldilocks area between about 2.4 billion years ago and about 1.4 billion years ago, where at 2.4 billion years ago, we started to be able to concentrate uranium on Earth.
• 22:27
  SPEAKER_01: And after 1.4 billion years ago, there wasn't enough 235 uranium around to sustain a chain reaction.
• 22:34
  SPEAKER_01: So there's really this Goldilocks window where natural nuclear reactors could occur.
• 22:38
  SPEAKER_01: And this was actually theorized in like the 1950s, uh, but we didn't discover a real fossil natural nuclear reactor until the the 70s.
• 22:48
  SPEAKER_00: Right.
• 22:48
  SPEAKER_00: So just to just to summarize again, then we we needed this recipe of oxygen and enough uranium to allow it to concentrate in, I think it what sandstone, right?
• 23:01
  SPEAKER_01: Yeah, exactly.
• 23:02
  SPEAKER_00: Groundwater moved it and concentrated it in sandstone.
• 23:04
  SPEAKER_01: And this is a way that a lot of uranium deposits are formed when we actually mine uranium to put to enrich and to put into nuclear reactors.
• 23:12
  SPEAKER_01: There's a lot of uranium in these sandstone type deposits.
• 23:16
  SPEAKER_00: It's just sitting where?
• 23:17
  SPEAKER_00: In the pore space?
• 23:18
  SPEAKER_00: Like in the spaces between the grains?
• 23:21
  SPEAKER_01: Yeah, basically you have this groundwater which has dissolved uranium in it, and then it hits basically a chemical front, a chemical boundary that changes the composition of the water such that uranium is no longer stable in the water and it gets dropped out.
• 23:34
  SPEAKER_01: And so it constantly basically the fluid is flowing through, hits this chemical boundary, and it just dumps all the uranium out all in one spot.
• 23:42
  SPEAKER_01: And so this creates a uranium deposit, and back in time, it created a uranium deposit that could sustain a nuclear reaction.
• 23:49
  SPEAKER_00: Okay, so then what happens next?
• 23:50
  SPEAKER_00: So we have this concentration of uranium.
• 23:53
  SPEAKER_01: Um, then what else do we need?
• 23:55
  SPEAKER_01: Yeah, so this comes back to our silly name, our moderator.
• 23:58
  SPEAKER_01: Yeah, there we go.
• 23:59
  SPEAKER_01: The moderator that moderates neutrons, not the reaction.
• 24:02
  SPEAKER_01: It's actually required to drive the reaction.
• 24:04
  SPEAKER_01: So we need something else there to slow down the neutrons.
• 24:08
  SPEAKER_01: Remember, 235 uranium does not absorb fast neutrons, it absorbs slow neutrons.
• 24:13
  SPEAKER_01: So we need to slow the neutrons down.
• 24:15
  SPEAKER_01: And basically the way this works is just have a light element around that the neutrons can ricochet off of, which slows them down.
• 24:21
  SPEAKER_01: So they're just bouncing off of stuff.
• 24:23
  SPEAKER_00: And so water does this, then, right?
• 24:24
  SPEAKER_01: Water does this exactly.
• 24:26
  SPEAKER_01: Graphite does this, other things do this.
• 24:28
  SPEAKER_01: But in this case, in this ancient 1.7 billion year old nuclear reactor, water was what was moderating these neutrons.
• 24:36
  SPEAKER_01: So we needed to have water around too.
• 24:38
  SPEAKER_00: All right.
• 24:38
  SPEAKER_00: So then you have the water, you have enough uranium because of the actions present in the atmosphere.
• 24:45
  SPEAKER_00: So, Jesse, we need to paint a picture of what this looked like then.
• 24:50
  SPEAKER_00: Can you walk us through that a second?
• 24:52
  SPEAKER_00: Like what okay, we had this these nuclear reactions.
• 24:56
  SPEAKER_00: If you were there 1.7 billion years ago watching this, what would it look like?
• 25:01
  SPEAKER_01: Yeah, great question.
• 25:02
  SPEAKER_01: I mean, these are not bombs, these aren't things that are blowing up.
• 25:04
  SPEAKER_01: That's first and foremost.
• 25:06
  SPEAKER_01: Um, because they're not compressed like a bomb is, you know, you don't have uranium packed in so tightly.
• 25:11
  SPEAKER_01: But they are very much like a nuclear power plant today.
• 25:15
  SPEAKER_01: And so these are little areas, the things in in Africa that these ancient nuclear reactors in, they're small areas.
• 25:22
  SPEAKER_01: Most of them are on average about 30 feet by like two feet thick.
• 25:26
  SPEAKER_01: So they're these relatively small areas that have concentrated uranium, and basically they start the fission reaction going.
• 25:33
  SPEAKER_01: And what does that do?
• 25:34
  SPEAKER_01: That heats up.
• 25:35
  SPEAKER_01: And so, just like a man-made nuclear power plant today, we got to get rid of that heat somehow.
• 25:41
  SPEAKER_01: Nuclear power plants use that heat to drive turbines and generate electricity.
• 25:45
  SPEAKER_01: Back then, what happened?
• 25:47
  SPEAKER_01: That heat just built up and it evaporated the water around it.
• 25:51
  SPEAKER_01: And this comes into the old faithful analogy of these nuclear reactors, which is so cool.
• 25:56
  SPEAKER_00: This is awesome.
• 25:57
  SPEAKER_00: So, basically, then what you're saying is that the heat evaporated the water, it created steam, which then rose up and it basically erupted, not like a bomb, but erupted like a geyser at the surface.
• 26:09
  SPEAKER_00: So we would have these geysers that were going off for about a half hour, right?
• 26:14
  SPEAKER_01: Right.
• 26:15
  SPEAKER_01: And so basically, these things you lose your water, so the water boils off, uh, and then you don't have the neutron moderator around.
• 26:23
  SPEAKER_01: So what happens?
• 26:24
  SPEAKER_01: The reaction slows down, which what happens?
• 26:26
  SPEAKER_01: It cools down that area because there's no nuclear reaction going on anymore.
• 26:31
  SPEAKER_01: So water seeps back in, and then that water helps moderate the neutrons, helps speed up this reaction, and then the water heats up and it boils off, and then it slows back down.
• 26:42
  SPEAKER_01: And we so we had this cycling on and off, on and off, on and off.
• 26:46
  SPEAKER_00: Gosh, that is that is amazing.
• 26:49
  SPEAKER_00: Uh, it really is so cool.
• 26:51
  SPEAKER_00: It's it's just like a geyser in Yellowstone.
• 26:53
  SPEAKER_00: In fact, it's very comparable, even in the time frame here that we're talking about, to old faithful.
• 26:59
  SPEAKER_00: Uh, but it is so same thing going on, just a totally different mechanism.
• 27:03
  SPEAKER_01: Exactly.
• 27:03
  SPEAKER_01: And so we had these cycles of about 30 minutes of a fission chain reaction, and then about two and a half hours of downtime for it to cool down and water to seep back in.
• 27:12
  SPEAKER_01: And in this little area, in this several square kilometers in Africa, uh, we had these small reactors, like I said before, about 30 feet by two feet uh deep, something like that.
• 27:22
  SPEAKER_01: And there were dozens of them around.
• 27:23
  SPEAKER_01: So these things were kind of happening all over the place.
• 27:25
  SPEAKER_01: And so did it, did it look like an erupting geyser?
• 27:28
  SPEAKER_01: I'm not sure if it would have been like a geyser or just your steam coming out of the ground, you know what I mean?
• 27:33
  SPEAKER_01: Like I'm not sure if it would have been explosive in the way that a geyser is.
• 27:36
  SPEAKER_00: But so I have a question then, and I think everybody listening to this has the same question.
• 27:41
  SPEAKER_00: Uh how how in the world are we able to determine that these events lasted 30 minutes?
• 27:47
  SPEAKER_01: Yeah.
• 27:47
  SPEAKER_00: Then you had two and a half hours of of downtime for it to like kind of regenerate and replenish, and then it happened again, and this went on for several hundred thousand years.
• 27:56
  SPEAKER_01: So it's a really this is an important question, and it highlights again two important aspects of the geosciences and uh the nuclear physics community in that when this uranium-235 breaks apart, it breaks apart into several different types of elements, and the types of elements that are produced in these nuclear reactors, so nuclear reactors are producing new elements because 235 uranium is fragmenting apart into different other elements.
• 28:20
  SPEAKER_01: And the way that those things fragment is dependent upon several parts of the nuclear reactor, meaning it's dependent upon the total mass of the uranium in the reactor, the amount of 235 uranium, the rate of the reaction, the type of moderator, et cetera, et cetera.
• 28:37
  SPEAKER_01: But it means that we can look at the like barium or neodymium or krypton, other elements in the nuclear reactor, and kind of infer the rate of the processes that were going on, the rate of the reaction that was going on at that time.
• 28:52
  SPEAKER_01: And so this is another really big industry in the nuclear forensics in the national laboratories is modeling nuclear reactors.
• 29:01
  SPEAKER_01: A lot of the like most powerful supercomputers on earth were designed specifically to model nuclear reactors.
• 29:08
  SPEAKER_01: And so we can apply those same reactor, you know, those same models to these ancient natural nuclear reactors and kind of work out this time scale that we've been talking about.
• 29:16
  SPEAKER_00: You just you went full on nerd there with that.
• 29:19
  SPEAKER_00: Yeah, I know.
• 29:20
  SPEAKER_00: Okay, I got a little bit carried away.
• 29:21
  SPEAKER_00: Sorry about that.
• 29:22
  SPEAKER_00: I got a little bit too excited.
• 29:23
  SPEAKER_00: So, I mean, holy cow.
• 29:25
  SPEAKER_00: So can I summarize maybe?
• 29:26
  SPEAKER_00: Yes, please do.
• 29:28
  SPEAKER_00: All right.
• 29:28
  SPEAKER_00: That um, wow, uh, I don't even know where to begin, actually, with your rambling uh should I just leave?
• 29:35
  SPEAKER_00: Should I just leave?
• 29:36
  SPEAKER_00: You can get somebody else to run this podcast with you.
• 29:40
  SPEAKER_00: Um, so basically, we're able to determine how long these reactions took place by the isotopes that are still present within these rocks, right?
• 29:52
  SPEAKER_00: And these isotopes are created by the fission of uranium-235.
• 29:57
  SPEAKER_00: And yep, that's a much better summary of the case.
• 30:01
  SPEAKER_00: That's probably enough.
• 30:02
  SPEAKER_01: Yeah, that's probably enough.
• 30:03
  SPEAKER_01: That's enough.
• 30:04
  SPEAKER_00: Okay, I mean, so there's basically we basically we know which I think is amazing.
• 30:09
  SPEAKER_00: I I actually do.
• 30:10
  SPEAKER_00: I think that these kinds of studies that people do is truly uh amazing.
• 30:17
  SPEAKER_00: They're studying these things, they're answering these questions, and and um they may not really understand why I am doing this research, what questions am I going to answer?
• 30:27
  SPEAKER_00: They don't know that, and then something like this comes along and it applies to this very important and and cool uh factoid in geology.
• 30:38
  SPEAKER_01: And to build off that, it really highlights the usefulness of these ancient natural nuclear reactors for informing us about nuclear processes on Earth and uh how we go about generating nuclear energy, because there's a few kind of takeaways here is that first of all, these fission products, these radioactive fission products, have kind of stuck around this region.
• 30:56
  SPEAKER_01: So there's some lessons to be learned in how to store nuclear waste that can be taken away from these ancient nuclear reactors by looking at the minerals present in these deposits and all that kind of stuff.
• 31:06
  SPEAKER_01: The other thing that this has taught us a little bit is how to create safe nuclear reactors, because these were safe nuclear reactors.
• 31:14
  SPEAKER_01: They did not overheat, they did not melt down, they didn't create a bomb, they just boiled off their moderators.
• 31:19
  SPEAKER_01: So they're actually nuclear reactor designs, modern nuclear reactor designs, that are in some ways based off of the principles of these fossil ancient nuclear reactors from two billion years ago.
• 31:30
  SPEAKER_00: Hey, I have a question though.
• 31:31
  SPEAKER_00: Um, does so these isotopes that were created by the fission and still are stored in the rocks, does this have any implications for storing nuclear waste today?
• 31:45
  SPEAKER_01: Yeah, we can learn how we can look at different minerals that were formed two billion years ago in these ancient reactors and are still present and still holding the nuclear fission products.
• 31:56
  SPEAKER_01: So, absolutely, there are absolutely lessons to be learned about how to safely store nuclear waste for billions of years.
• 32:03
  SPEAKER_01: Um, this is an active area of research for many people.
• 32:06
  SPEAKER_00: Wow, that's really cool.
• 32:07
  SPEAKER_00: Okay.
• 32:08
  SPEAKER_00: Um, so I think we should go ahead and wrap this episode up, Jesse.
• 32:12
  SPEAKER_00: Um, you know, ultimately this story highlights how geoscience is intimately linked to human energy production and specifically how the nuclear physics and Manhattan Project contributed to our understanding of the earth.
• 32:26
  SPEAKER_00: It's just a very, very cool story.
• 32:28
  SPEAKER_00: I'm gonna go officially on record as saying um you were right, I was wrong.
• 32:34
  SPEAKER_01: Well, well, thank you for uh for being can be able to be convinced or allowing yourself to be convinced.
• 32:39
  SPEAKER_01: It took some doing, but I think it's super cool.
• 32:42
  SPEAKER_01: It's near and dear to to my heart, in part because I I've worked for many years on designing these types of instruments that make these measurements, and it's just a really cool representation of of how geoscience is important for our modern society.
• 32:56
  SPEAKER_00: I I agree wholeheartedly, and you know, I'm gonna tell you right now, the tipping point for me was when you said this is the coolest factoid in geology that I have.
• 33:05
  SPEAKER_00: And and you followed that up and said, What's yours?
• 33:08
  SPEAKER_00: Yeah, and I don't I don't have one.
• 33:10
  SPEAKER_01: I thought about this uh like I'm embarrassed for you, and I'm embarrassed because of you actually now.
• 33:17
  SPEAKER_01: So um you're gonna have to come up with one because I know I'm gonna put you on the spot.
• 33:21
  SPEAKER_01: At some point, we're gonna sit down, I'm gonna put you on the spot with a hot mic, and you're gonna have to come up with one.
• 33:26
  SPEAKER_01: So fair game.
• 33:28
  SPEAKER_00: Yeah, I I uh I don't know.
• 33:30
  SPEAKER_00: I actually have racked my brain over this.
• 33:33
  SPEAKER_00: I have these moments in my life, you you know, I recall falling in love with geology and and all that.
• 33:39
  SPEAKER_00: I I have that, and and I have these moments where I I fall in love with it all over again, yeah, but it's based on other things, it's not uh I don't have right now off the top of my head, which I have actually thought a lot about, so it's not off the top of my head.
• 33:55
  SPEAKER_00: I don't have an example.
• 33:56
  SPEAKER_00: I don't know.
• 33:57
  SPEAKER_00: It's weird.
• 33:58
  SPEAKER_01: What the hell am I doing talking to you?
• 34:00
  SPEAKER_01: Unbelievable.
• 34:02
  SPEAKER_01: Why do I associate with you?
• 34:03
  SPEAKER_01: It's unbelievable.
• 34:04
  SPEAKER_01: My goodness.
• 34:05
  SPEAKER_01: I'm gonna have to rethink my life right now.
• 34:07
  SPEAKER_01: You are unbelievable.
• 34:08
  SPEAKER_01: All right, Chris.
• 34:09
  SPEAKER_01: Well, this has been fun.
• 34:10
  SPEAKER_01: Thanks for uh being a willing participant in this.
• 34:13
  SPEAKER_01: Uh you bet.
• 34:14
  SPEAKER_01: It was awesome.
• 34:14
  SPEAKER_00: I love it.
• 34:15
  SPEAKER_01: Super fun stuff.
• 34:16
  SPEAKER_01: All right, and as usual, you can find us on Facebook, Instagram, Twitter.
• 34:19
  SPEAKER_01: We are at Planet Geocast.
• 34:21
  SPEAKER_01: Send us your questions, send us your comments if you have episode ideas, interesting people to interview, hit us up with that stuff.
• 34:28
  SPEAKER_01: Our email is at planetgeocast at gmail.com.
• 34:31
  SPEAKER_01: Take care, everybody.
• 34:33
  SPEAKER_01: Have a good week.