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Nature's Nuclear Reactors

Article #27 • Written by Alan Bellows

▼ Scroll to Continue ▼

In the early- to mid-1950s, Dr. Paul Kuroda from the University of Arkansas described the possibility of naturally occurring nuclear reactors lurking in the crust of ancient Earth. The key is an isotope of Uranium called U-235, which occurs naturally in small amounts. If enough of this isotope were pooled together under specific circumstances, Kuroda theorized, the natural reactor would go critical, and self-sustaining fission would occur. Such a reactor could not exist today, because too much of the Earth's natural U-235 has decayed... but a billion and a half years ago, there was enough of it around to make the idea plausible. In point of fact, it has since been discovered that it actually happened.

In 1972, the radioactive remains of such a nuclear reactor was found in the state of Gabon in West Africa, in the Oklo mines. Uranium extracted from that mine was abnormally short in U-235 isotopes, and upon examination, French scientists found that the uranium isotope levels had an uncanny resemblance to those in spent nuclear fuel from modern nuclear power plants. The evidence was strong enough to suggest a natural reactor, and further exploration confirmed it.

At the time of discovery, scientists were uncertain exactly how the Oklo reactor had operated without exploding or melting down. For 150 million years, it ran like clockwork with a 30 minute reaction cycle, followed by a 2.5 hour cool-down cycle, putting out an average of 100 kilowatts of power. And it was always exactly 30 minutes per cycle, without significant variation, which was baffling. But recent studies have finally solved the mystery by discovering the regulating mechanism: Water.

Under normal conditions, radioactive atoms like U-235 cast off neutron particles at speeds so high that most of the neutrons skip off the surface of other atoms and fly away. But if you put enough of the radioactive material together, the cast-off neutrons bounce around inside the mass, some slowing down enough to be absorbed into another atom's nucleus. The extra neutron causes the nucleus to become unstable and immediately split, which releases a large amount of energy. If one has enough radioactive material in sufficient density that a lot of nuclei split very rapidly (critical mass), the reaction increases exponentially, and results in an atomic explosion. Any less than that (subcritical mass), and it causes a sustained fission reaction, giving off energy as heat and radiation.

But researchers have determined that Oklo didn't even have an appreciable, consolidated subcritical mass of Uranium... it was too spread out. Instead, water would seep down through crevices to fill up the gaps between the uranium deposits, and act as a "neutron moderator," slowing down the neutrons enough to allow them to hit-and-split other nuclei. When the reaction caused a sufficient heat increase, the water would boil off, removing the neutron moderator, and stop the process. The cavity would then slowly refill with water during the cooling period, starting the cycle again.

Fifteen such natural reactors have been found in the Oklo area, and they are now collectively referred to as the "Oklo Fossil Reactors." These natural reactors are providing useful data on long-term storage of spent nuclear fuel, as well as some insights into possible improvements in man-made reactors.

Article written by Alan Bellows, published on 11 October 2005. Alan is the founder/designer/head writer/managing editor of Damn Interesting.

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41 Comments
Bryan Lowder
Posted 12 October 2005 at 05:53 pm

Very cool, Alan. I read about this first in my Halliday & Resnik physics text, but it wasn't nearly in this detail.


hfswagon
Posted 02 December 2005 at 10:51 pm

I think you meant PROMPT CRITICAL when you wrote CRITICAL MASS. Critical mass is just the amount necessary to maintain the chain reaction. Prompt critical is when it reacts uncontrollably (as in an explosion).


sulkykid
Posted 26 May 2006 at 08:31 am

I cannot see how that scientists would know that the reactor would work for 30 minutes then cool down for 150 minutes. when this happens millions of years ago.


Secret Ninja
Posted 08 August 2006 at 07:58 pm

Science, Blah.

I say WITCHCRAFT!


just_dave
Posted 08 August 2006 at 08:26 pm

Secret Ninja said: "Science, Blah.

I say WITCHCRAFT!"

Nah; I say alien visitors, or some advanced ancient civilization. Who knows; maybe it was the reactor that did them in, whoever they were.

______

Speaking of nuclear reactors, anybody remember the old SNL skit about a guy retiring from his job at the nuclear power plant? His last words to the much younger crew was, "Remember; you can't put too much water in a nuclear reactor!"


Misfit7707
Posted 08 August 2006 at 11:36 pm

It were the monkeys! I'z seen 'em!!

If it's possible for a nuclear reaction to take place, A) I would, along with sulkykid, like to know how the scientists knew that the time frame for the cycle was 30 minutes exactly, and B) Is it possible at all for a nuclear meltdown, or as hfswagon put it, PROMPT CRITICAL? Certainly the water here prevented anything from heating up like that, but COULD nature ever make something like that?

Maybe the dinosaurs were killed off by Mother Nature's dirty bomb.


totoro!totoro!
Posted 09 August 2006 at 12:01 am

how fascinating. and i found the explanation of how an atomic explosion starts terribly edifying. i always wondered what happens but i was too shy to ask.
damninteresting.com is one of my favourite websites because of articles like these.


mensadave
Posted 09 August 2006 at 05:03 am

Aha! So that explains why the dinosaurs died off and the mutant evil homo sapiens gained dominance over the world!


Johnny Wayne
Posted 09 August 2006 at 07:04 am

The last time I dug myself a nuclear reactor pit, I couldn't get it to work. Seems I was missing the water part. I'm getting out the garden hose now.


Johnny Wayne
Posted 09 August 2006 at 08:56 am

I just had one more thought. Depending on the timing of these reactors, I wonder if they had an impact on the development of life. What sort of life was around a billion and a half years ago? Was it single cell stuff? Could radiation from fission have had an impact on allowing for more complicated lifeform structures? Now I have to go dig up the timeline for the development of life on earth and see if there are any correlations. hmmmm


Rinson Drei
Posted 09 August 2006 at 09:19 am

Am I the only one upset at Mother Nature for wasting so much of our precious uranium? sheesh


SparkyTWP
Posted 09 August 2006 at 09:45 am

From the wikipedia article:

"The fact that all five known normally produced isotopes from fission of the fission-product gas xenon have been found in certain concentrations trapped in the remnants of the natural reactor points to on-again off-again reactor operation. The specific concentrations of xenon found trapped in mineral formations 2 billion years after natural reactor operation make it possible to calculate the specific time intervals the reactor would operate for: approximately 2.5 hours."

I'm not that familiar with nuclear science so I don't know how that exactly tells them the period, much less the 30 minute "on" cycle, but I guess it's at least something for an answer.


Stead311
Posted 09 August 2006 at 10:22 am

Isn't it dangerous to be around there? Wouldn't there be spent nuclear fule or waste, something of that nature. I am pretty ignorant when it naive when it comes to anything nuclear. All I know is that I am surprised that russia doesnt own that part of Africa already to scoop up whats left.


Stead311
Posted 09 August 2006 at 10:23 am

Yea i can't type today. sorry folks.


frenchsnake
Posted 09 August 2006 at 10:37 am

As the nuclear reactions in the Oslo mines seem to have happened over a billion years ago, maybe whatever nuclear waste there would be would have broken down by now? Not that I know anything about nuclear physics. Oh, and any idea why the nuclear reactors in the Oslo mines stopped working? Was it a gradual decay, or did it stop suddenly? With everything they're discovered about them, it seems they should know that too.


NastyGash
Posted 09 August 2006 at 11:17 am

This storry could not possible be factal since the EARTH is ONLY 10,000 (aprox) year's old. Dont you guy's reed the Bibble?


NastyGash
Posted 09 August 2006 at 11:18 am

Soory. That's should be BIBLE.


yazheirx
Posted 09 August 2006 at 11:20 am

SparyTWP:

When a nuclear fission happens it creates two or more new atoms. There is a significant, and known, chance for one of the new atoms to be Xenon.

So given the concentration of Xenon compared to the other fission products you can tell how quickly it was created.

Xenon accepts neutrons readily, this reduces the number of neutrons available to cause fission. That combined with the removal of the moderator (a substance that slows down neutrons so that they are going slow enough to be captured by an atom (think of refuelling on a highway if the tanker is doing 90 kph and you are doing 40 kph nothing, other than a large mess, will be transferred) causes the reaction to stop or slow down.

If you are following all the above logic then you can take the rather complex algorithms that describe a nuclear reaction and use them to determine things like the rate at which fission occurred, how long it occurred, and how long the "reset" time was. This math is used by the US Navy when their reactors near end of life and they may be "Xenon Precluded" from starting up again.


Stead311
Posted 09 August 2006 at 11:34 am

Isn't it dangerous to be around there? Wouldn't there be spent nuclear fule or waste, something of that nature. I am pretty naive when it comes to anything nuclear. All I know is that I am surprised that russia doesnt own that part of Africa already to scoop up whats left.


flstc2000
Posted 09 August 2006 at 01:56 pm

NastyGash said: "This storry could not possible be factal since the EARTH is ONLY 10,000 (aprox) year's old. Dont you guy's reed the Bibble?"

Actually, this Earth that was created 10,000 years ago could have been formed from pieces of existing matter that were indeed millions and or billions of years old.


Killtrocity
Posted 09 August 2006 at 03:23 pm

This is very cool,and makes one wonder if it would be possible just to re-make something like this just for the ease of use for power.


joe schmoe
Posted 09 August 2006 at 05:14 pm

NastyGash said: "This storry could not possible be factal since the EARTH is ONLY 10,000 (aprox) year's old. Dont you guy's reed the Bibble?"

Genesis 1:1 In the beginning God created the heaven and the earth.
Genesis 1:2 And the earth was(orginal hebrew is "became") without form and void:

I ask when was the beginning? Are any of us qualified to guess? Of course the earth is billions or rather eons old. It is MAN who sets the age of the world at 10K, 6K, or what ever not God's word.


another viewpoint
Posted 10 August 2006 at 05:24 am

...leave it to the French to dig themselves into hole only to find...nothing of any value.

btw (interesting math problem)...does anyone out there in Damn Interesting land know how much earth can be removed from a hole that is, say... 2 feet long, 2 feet wide and 2 feet deep? Stay tuned...


irea6242
Posted 10 August 2006 at 06:37 am

I wonder how much it would cost to dig pits that deep in order to transport current nuclear waste and then re-bury it.

Or instead, perhaps the ore around those reactors has special radiation-absorption properties that make it a great envelope?


Johnny Wayne
Posted 10 August 2006 at 07:19 am

sure, 10,000 years old. in God years.

another viewpoint, I've done many trials on this. I've constructed elaborate measurement devices, and escavating equipment, specific to test your problem. I even built a time machine, so I could do 5 years of testing, and come back to today and provide an immediate answer. If I start with your 2 foot by 2 foot by 2 foot hole, and begin removing land from it, well, after 5 long years, I must report complete failure. I have been unsuccessful, failing to remove even a thimble full of land. My apologies.


another viewpoint
Posted 10 August 2006 at 09:16 am

JW...no failure to report. You did good...'cause you can't remove dirt from a hole that's already empty! You're a winner...you can collect your cookie anytime.


skeptic
Posted 10 August 2006 at 08:19 pm

The rock surrounding the Oklo reactors is nothing special, yet studies of Oklo reveal that the radiactive waste, or "fission products", only migrated through these rocks for just a few inches in a billion+ years. Yet anti-nucs say the exact same elements created in modern power reactors should not be buried because the fission products will travel through miles of similar rock in just thousands of years and so kill our descendents. Mother Nature proves them yet wrong again.


irea6242
Posted 11 August 2006 at 05:31 am

Perhaps they were thinking we'd be fool enough to store the waste under live volcanoes. :)


tza
Posted 12 August 2006 at 11:45 am

Stead311 said: "All I know is that I am surprised that russia doesnt own that part of Africa already to scoop up whats left."

You're an ignorant a$$hole. Looks like you don't know much dikhead. Its interesting that Russians dont talk sht about Americans, because we have a level of interest and respect (for all people), whereas all you know is blatant discrimination. Quit living on bad blood you child.

NastyGash said: "This storry could not possible be factal since the EARTH is ONLY 10,000 (aprox) year's old. Dont you guy's reed the Bibble?"

That's quite the naive statement to make seeing how there is no 'factal' proof the existance of a god who created this planet 10k years ago. I would think that any rational human would stick to true scientific facts that can take a substance with a known half-life, and estimate (within a thousand years even) how old it is. To believe that nothing on this planet is older then 10k years because god, with his magical wand, created it then is blatantly obsurd. Face the facts - the thing you call god is nothing but a story to passify the masses and justify wars for land (just like most other things). Not to say that there is no other mystical creature out there (alien or other) that has aided in our evolution, but I highly doubt they care as to how often you go to church or if you say your prayers at night.
Don't believe science fully, but have a certain level of objectivity to everything otherwise your just another brainwashed fool roaming this here planet.
There was an article I read a while back about what looked like a nuclear waste dump somewhere in Nevada. It was also post-fissioned (waste from nuclear reactors) U235, but in huge amounts buried and dated back some 2 billion years. The was an accompanying theory (of course) which said that it is very possible that man-kind has roamed the world multiple times now, destroyed itself and/or left this planet for something better. We're a class 0 civilization - we haven't figured out a way to use any of the energy in our solar system besides whats on our planet.. Woulda loved to meet those kids guys.


911review
Posted 15 August 2006 at 08:31 pm

NastyGash said: "This storry could not possible be factal since the EARTH is ONLY 10,000 (aprox) year's old. Dont you guy's reed the Bibble?"

Actually, this Earth that was created 10,000 years ago could have been formed from pieces of existing matter that were indeed millions and or billions of years old.

yea, and all those million year old dinosaur bones are just dust that just happen to form into bones

by the way, you cant get dirt out of a hole.
(definition of hole, is the absence of whatever matter was there before you created it)
Brad
911review.org


rev.felix
Posted 05 January 2007 at 10:46 am

tza said: "You're an ignorant a$$hole. Looks like you don't know much dikhead. Its interesting that Russians dont talk sht about Americans, because we have a level of interest and respect (for all people), whereas all you know is blatant discrimination. Quit living on bad blood you child.

Right, you really seem to have respect for the people you bash.

As to the 10K vs. Eons argument, it can easily be solved by taking into acount relitivistic effects (yes i know i can't spell), it all depends on your inertial frame of referance.


kgy121
Posted 04 June 2007 at 07:30 pm

another viewpoint said: "…does anyone out there in Damn Interesting land know how much earth can be removed from a hole that is, say… 2 feet long, 2 feet wide and 2 feet deep? Stay tuned…"

None. A hole is already empty. :P


Kao_Valin
Posted 12 December 2007 at 02:02 pm

NastyGash said: "Soory. That's should be BIBLE."

Yes, you better correct yourself. The Bibble says its 20 thousand years old, because the Bibble is twice as good as the Bible. Older and wiser the Earth be, I say!


Nezbitz
Posted 06 February 2008 at 08:55 pm

Kao_Valin said: "Yes, you better correct yourself. The Bibble says its 20 thousand years old, because the Bibble is twice as good as the Bible. Older and wiser the Earth be, I say!"

Two B or not Two B. That is the question :P


Mirage_GSM
Posted 05 August 2008 at 04:38 am

flstc2000 said: "Actually, this Earth that was created 10,000 years ago could have been formed from pieces of existing matter that were indeed millions and or billions of years old."

Cool! I can imagine God sitting there with a jigsaw of parts, clobbering together earth: "Hmm. Gabon... That's the thing with tha old nuclear reactor I built as a kid. Let's put it between Cameroon and Congo!
And regarding possible remaining nuclear waste: After 1,5 billion years you could probably handle a spent fuel rod from an atomic power plant with your bare hands without any ill effects.


Nick
Posted 25 September 2008 at 09:09 am

.........maybe 400 million earth years ago, (just b4 dinosaurs became), this bygone civilization that had nuclear energy, & whatnot, blow themselves up in war or a big accident like a gigantic melt-down occurred, after this happened, years later or 100's or 1000's years later, reptiles became bigger becuase of radiation & there we have why dinosaurs were so large! just a thought, ( like the movie Godzilla) the intelligent beings that ruled were knocked down to just a few 100 after this event, (& lived underground) & were made sterile, & died out.


ValiantDefender
Posted 25 November 2008 at 12:22 pm

NastyGash said: "This storry could not possible be factal since the EARTH is ONLY 10,000 (aprox) year's old. Dont you guy's reed the Bibble?"

I know you're merely poking fun but a few questions about this. According to the Doctrine of my church, the Earth was Organized from materials. In other words, god didn't "create" as in poof into existance, a planet. Rather, took of raw materials and created the earth. Much like a baker "makes" a cake. The cake is only a few minutes old, all toasty warm and moist. But the wheat used to make it 30 years old (there is some 30 year old wheat in my basement...I know, it has no nutritional value left). So, while the religious right contends the Earth is only 10k years old, the materials used to make it may be much, much older. This resolves many divergences between the religious point of view and the scientific point of view.

I'm not saying you have to agree with me or that my view supercedes yours. Just asking for common courtesy.

I think the article is great. I wish the site was still adding new stuff. It seems like the book is now the 100% focus. Dunno why,lol. It probably makes money =)


Jospec5Star
Posted 17 January 2009 at 03:58 pm

Damn interesting indeed. I would say that prior to finding this site I was in no way interested in radioactive material or the like. However the writing style and stories themselves have completely changed that for me. Excellent job DI staff!


Hermes
Posted 27 July 2009 at 11:13 am

(better late than never) - In reading over these comments I can't help but notice a fairly general misunderstanding of the difference between this phenomenon and intentionally built reactors. I see a lot of speculation regarding "how do they know...", "but what about...", etc.
Here is a link to an excellent and detailed article from Scientific American magazine a while back written by those directly involved in the research:
http://www.scientificamerican.com/article.cfm?id=ancient-nuclear-reactor
If you are interested in the science of this, it is a fascinating story about how they figured all this out. Of note: The total power output this reactor was "say, enough to run a few dozen toasters" (had it been harnessed to make elctricity, and if there were toasters a billion years ago) - this is no Chernobyl.
The by-products are no threat, since the longest-lasting radioactive by-products from a U-235 reaction are dangerous only for about 100,000 years, so while that time frame may pose an issue for reactors operating within our lifetimes, 10 times that period oftime has elapsed since these natural reactors stopped.
As for the possibility of using such things to make electricity, that ship has sailed. the proportion of U-235 among all uranium is the same everywhere in our solar system (except at the Oklo sites - where there is less of it, because the natural reaction used it up), and that proportion keeps decreasing over time (due to natural radioactive decay into other isotopes that can't sustain a reaction). In the current era, the proportion is already too small for this to happen again. That is why power plants have to use "enriched" uranium, which is uranium processed to increase its U-235 proportion so that it can work.

Perhaps more info than anyone wanted, but hey, this is my first post here (bash away!)


Mullin
Posted 27 July 2009 at 06:08 pm

Wow, great post Hermes.
These are the types of comments that actually make the site better, hopefully not your last post.


TravelBugBrit
Posted 19 October 2010 at 11:09 pm

Agreed


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