Uranium ore.
Image: United States Geological Survey and the Mineral Information Institute
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Editor's Note: This article originally appeared in the October 2005 issue of Scientific American.
In May 1972 a worker at a nuclear fuel–processing plant in France noticed something suspicious. He had been conducting a routine analysis of uranium derived from a seemingly ordinary source of ore. As is the case with all natural uranium, the material under study contained three isotopes— that is to say, three forms with differing atomic masses: uranium 238, the most abundant variety; uranium 234, the rarest; and uranium 235, the isotope that is coveted because it can sustain a nuclear chain reaction. Elsewhere in the earth’s crust, on the moon and even in meteorites, uranium 235 atoms make up 0.720 percent of the total. But in these samples, which came from the Oklo deposit in Gabon (a former French colony in west equatorial Africa), uranium 235 constituted just 0.717 percent. That tiny discrepancy was enough to alert French scientists that something strange had happened. Further analyses showed that ore from at least one part of the mine was far short on uranium 235: some 200 kilograms appeared to be missing— enough to make half a dozen or so nuclear bombs.
For weeks, specialists at the French Atomic Energy Commission (CEA) remained perplexed. The answer came only when someone recalled a prediction published 19 years earlier. In 1953 George W. Wetherill of the University of California at Los Angeles and Mark G. Inghram of the University of Chicago pointed out that some uranium deposits might have once operated as natural versions of the nuclear fission reactors that were then becoming popular. Shortly thereafter, Paul K. Kuroda, a chemist from the University of Arkansas, calculated what it would take for a uraniumore body spontaneously to undergo selfsustained fission. In this process, a stray neutron causes a uranium 235 nucleus to split, which gives off more neutrons, causing others of these atoms to break apart in a nuclear chain reaction.
Kuroda’s first condition was that the size of the uranium deposit should exceed the average length that fission-inducing neutrons travel, about two thirds of a meter. This requirement helps to ensure that the neutrons given off by one fissioning nucleus are absorbed by another before escaping from the uranium vein.
A second prerequisite is that uranium 235 must be present in sufficient abundance. Today even the most massive and concentrated uranium deposit cannot become a nuclear reactor, because the uranium 235 concentration, at less than 1 percent, is just too low. But this isotope is radioactive and decays about six times faster than does uranium 238, which indicates that the fissile fraction was much higher in the distant past. For example, two billion years ago (about when the Oklo deposit formed) uranium 235 must have constituted approximately 3 percent, which is roughly the level provided artificially in the enriched uranium used to fuel most nuclear power stations.
The third important ingredient is a neutron “moderator,” a substance that can slow the neutrons given off when a uranium nucleus splits so that they are more apt to induce other uranium nuclei to break apart. Finally, there should be no significant amounts of boron, lithium or other so-called poisons, which absorb neutrons and would thus bring any nuclear reaction to a swift halt.
Amazingly, the actual conditions that prevailed two billion years ago in what researchers eventually determined to be 16 separate areas within the Oklo and adjacent Okelobondo uranium mines were very close to what Kuroda outlined. These zones were all identified decades ago. But only recently did my colleagues and I finally clarify major details of what exactly went on inside one of those ancient reactors.
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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