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 (U) deposits might have once operated as natural versions of the nuclear fission reactors. Shortly thereafter, Paul K. Kuroda, a chemist from the University of Arkansas, calculated what it would take for a uranium ore body spontaneously to undergo self-sustained fission. The size of the uranium deposit should exceed two thirds of a meter, the average length that fission-inducing neutrons travel. Also, U-235 must be present in sufficient abundance approximately 3%.
The Oklo Fossil Fission Reactors, various separate areas within the Oklo and adjacent Okelobondo uranium mines, is one of the most fascinating stories in the relatively short history of science and especially in the even shorter history of Nuclear Physics. In 1972 the very well preserved remains of several ancient natural nuclear reactors were discovered in the middle of the Oklo Uranium ore deposit. Since their discovery the Oklo reactors have been studied by many scientists around the world. Two billion years ago (about when the Oklo deposit formed) U-235 must have constituted about 3% of these deposites, which is roughly the level provided artificially in the enriched uranium used to fuel most nuclear power stations. But only recently did Prof. Alex Meshik and his colleagues from Washington University, MO, USA finally clarify major details of what exactly went on inside one of those ancient reactors. Their article originally appeared in the October 2005 issue of Scientific American.
The basic idea that natural fission reactions were responsible for the depletion in U-235 at Oklo was accepted by scientists quite soon after the anomalous uranium was discovered. The high abundance of lighter elements created via fission proved an indisputable evidence for the same. Some of the neutrons released during the fission of U-235 were captured by the more abundant U-238 to produce U-239 which, after undergoing two beta decays, turned into plutonium (Pu)-239. More than two tons of this Pu isotope were generated within the Oklo deposit, most of which has decayed away over time.
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 did these “reactor cores” not explode right after nuclear chain reactions began? What was their self-regulating mechanism? Did these reactors run continuously or intermittently? The solutions of many puzzling aspects of these reactors came by only slowly over next few decades. The recent work by Prof. Alex Meshik and colleagues centred on an analysis of xenon (Xe), an inert gas product of fission, to find out what exactly went on inside these reactors. A couple of surprising results lead to deeper thinking about the mechanism and circumstances involved. The majority of Xe was not located in uranium-rich mineral grains but was mainly trapped in aluminum phosphate minerals, which contain no uranium at all. The second epiphany was the significantly different isotopic makeup of extracted gas as compared to that usually produced in nuclear reactors. The deeper analysis and contemplation revealed that the Xe isotopes measured resulted from decay of radioactive daughter products and not directly from U fission. Also, a key insight gained was that different xenon isotopes were created at different times depending on the half-lives of the parents and grandparents.
The Oklo reactors regulated themselves, most likely, through use of groundwater. Due to ongoing nuclear chain reaction, the temperature of the deposit would go on increasing over time. Once a critical temperature was reached, the water presumably boiled away. Without water present to act as a neutron moderator, the chain reaction would have temporarily ceased. Only after things cooled off and sufficient groundwater once again permeated the zone of reaction could fission resume. Large quantities of water must have been moving through these rocks, enough to wash away some of the xenon, tellurium and iodine, which are water-soluble. The attempted to model the process mathematically revealed much about the timing of reactor operation. The Oklo reactor would have switched “on” for 30 minutes and “off” for at least 2.5 hours.A similar zone of ancient nuclear fission was found in Bangombe, located some 35 kilometers away from Oklo. The Bangombe reactor is of special interest because it was more shallowly buried and thus has had more water moving through it in recent times. These ancient natural reactors could prove good models for studying the movement of nuclear waste from repositories. They may also teach scientists about possible shifts in what was formerly thought to be a fundamental physical constant, alpha, which controls such universal quantities as the speed of light.