Proof in the Light Elements
Physicists confirmed the basic idea that natural fission reactions were responsible for the depletion in uranium 235 at Oklo quite soon after the anomalous uranium was discovered. Indisputable proof came from an examination of the new, lighter elements created when a heavy nucleus is broken in two. The abundance of these fission products proved so high that no other conclusion could be drawn. A nuclear chain reaction very much like the one that Enrico Fermi and his colleagues famously demonstrated in 1942 had certainly taken place, all on its own and some two billion years before.
Shortly after this astonishing discovery, physicists from around the world studied the evidence for these natural nuclear reactors and came together to share their work on “the Oklo phenomenon” at a special 1975 conference held in Libreville, the capital of Gabon. The next year George A. Cowan, who represented the U.S. at that meeting (and who, incidentally, is one of the founders of the renowned Santa Fe Institute, where he is still affiliated), wrote an article for Scientific American [see “A Natural Fission Reactor,” by George A. Cowan, July 1976] in which he explained what scientists had surmised about the operation of these ancient reactors.
Cowan described, for example, how some of the neutrons released during the fission of uranium 235 were captured by the more abundant uranium 238, which became uranium 239 and, after emitting two electrons, turned into plutonium 239. More than two tons of this plutonium isotope were generated within the Oklo deposit. Although almost all this material, which has a 24,000-year halflife, has since disappeared (primarily through natural radioactive decay), some of the plutonium itself underwent fission, as attested by the presence of its characteristic fission products. The abundance of those lighter elements allowed scientists to deduce that fission reactions must have gone on for hundreds of thousands of years. From the amount of uranium 235 consumed, they calculated the total energy released, 15,000 megawatt-years, and from this and other evidence were able to work out the average power output, which was probably less than 100 kilowatts—say, enough to run a few dozen toasters.
It is truly amazing that more than a dozen natural reactors spontaneously sprang into existence and that they managed to maintain a modest power output for perhaps a few hundred millennia. Why is it that these parts of the deposit did not explode and destroy themselves right after nuclear chain reactions began? What mechanism provided the necessary self-regulation? Did these reactors run steadily or in fits and starts? The solutions to these puzzles emerged slowly after initial discovery of the Oklo phenomenon. Indeed, the last question lingered for more than three decades before my colleagues and I at Washington University in St. Louis began to address it by examining a piece of this enigmatic African ore.
Noble-Gas Epiphanies
Our recent work on one of the Oklo reactors centered on an analysis of xenon, a heavy inert gas, which can remain imprisoned within minerals for billions of years. Xenon possesses nine stable isotopes, produced in various proportions by different nuclear processes. Being a noble gas, it resists chemical bonding with other elements and is thus easy to purify for isotopic analysis. Xenon is extremely rare, which allows scientists to use it to detect and trace nuclear reactions, even those that occurred in primitive meteorites before the solar system came into existence.
To analyze the isotopic composition of xenon requires a mass spectrometer, an instrument that can separate atoms according to their atomic weight. I was fortunate to have access to an extremely accurate xenon mass spectrometer, one built by my Washington colleague Charles M. Hohenberg. But before using his apparatus, we had to extract the xenon from our sample. Scientists usually just heat the host material, often above the melting point, so that the rock loses its crystalline structure and cannot hold on to its hidden cache of xenon. To glean greater information about the genesis and retention of this gas, we adopted a more delicate approach called laser extraction, which releases xenon selectively from a single mineral grain, leaving adjacent areas intact.
See what we're tweeting about
13 Comments
Add CommentI believe that your time scale is incorrect (2 B Yr) as you did not account for the decay of the speed of light.
Reply | Report Abuse | Link to thisStrange that alchemists believed that transmutation of elements was being achieved within the earth and it turns out they were right. The same process that can turn lead into gold was occuring.
Reply | Report Abuse | Link to this100 kilowatts is more than enough to run every single appliance at the same time in 10 the most over automated house in this county
Reply | Report Abuse | Link to thisA well written review on an awe-inspiring discovery
Reply | Report Abuse | Link to thisQuite Interesting.These discoveries prove what is known is handful and what is unknown is a mighty ocean.
Reply | Report Abuse | Link to thisI've often wondered about the working of these reactors. Thanks for the elucidation!
Reply | Report Abuse | Link to thisCould the Russians have been running an experiment? I'm just sayin'
Reply | Report Abuse | Link to thisI find this simply amazing that this could happen. I mean, I see the evidence, and I don't doubt it happened, but imagine if anyone had tried to enter the cave while the fission was occurring. Ouch.
Reply | Report Abuse | Link to thisI have read many things about this kind of stuff, may be it is true in some part of Chariots Of The Gods, and i believe it
Reply | Report Abuse | Link to this"Since the advent of nuclear power generation, huge amounts of radioactive xenon 135, krypton 85 and other inert gases that nuclear plants generate have been released into the atmosphere." This is largely incorrect. Until reprocessing of the fuel takes place these fission product gases are locked into the ceramic fuel matrix inside the metallic cladding. During reprocessing the gases are captured and held for decay. In the US, since no reprocessing of commercial fuel has taken place for decades, the gases are still inside the fuel assemblies, slowly decayng off.
Reply | Report Abuse | Link to thisall that's left of the nuke materials are seen in the stars. What's left are scattered pieces when accumulate and collected by man will produce enough mass to activate the same energy of a nuclear reaction.
Reply | Report Abuse | Link to thisC-Decay has no substance and has been abandoned by creationists. The universe is older than 10,000 years get over it. Ironically, when I heard about the ancient reactors in Africa, I immediately thought about the first humans evolved out of Africa.
Reply | Report Abuse | Link to thisA little side agenda… We have enough nuclear power to have a nearly infinite supply of energy from reprocessing and breeder reactors… WITHOUT FUSION REACTORS… Let’s all be pro-nuclear and quit spending so much of our money on electricity. It’s truly the most ignorant thing since racism. They aren’t afraid of nuclear plants; they are afraid of what would happen if we allowed reprocessing and recycling…look it up
C-Decay has no substance and has been abandoned by creationists. The universe is older than 10,000 years get over it. Ironically, when I heard about the ancient reactors in Africa, I immediately thought about the first humans evolved out of Africa.
Reply | Report Abuse | Link to thisA little side agenda… We have enough nuclear power to have a nearly infinite supply of energy from reprocessing and breeder reactors… WITHOUT FUSION REACTORS… Let’s all be pro-nuclear and quit spending so much of our money on electricity. It’s truly the most ignorant thing since racism. They aren’t afraid of nuclear plants; they are afraid of what would happen if we allowed reprocessing and recycling…look it up