Two billion years ago, parts of an African uranium deposit spontaneously underwent nuclear fission. The details of this remarkable phenomenon are only just becoming clear.
In May 1972, while conducting a routine analysis of uranium derived from a seemingly ordinary source of ore — and, as is the case with all natural uranium, the material under study contained three isotopes, 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 — a French engineer named Bougzigues noticed something suspicious at a nuclear fuel-processing plant in France.
Elsewhere in the earth’s crust, on the moon and even in meteorites, uranium 235 atoms make up 0.720 per cent of the total. But in the French samples, which came from the Oklo deposit in Gabon (a former French colony in west equatorial Africa), uranium 235 constituted just 0.717 per cent. 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 kg appeared to be missing — enough to make half a dozen or so nuclear bombs.
For weeks, specialists at the French Atomic Energy Commission 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 uranium ore body spontaneously to undergo self-sustained 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 metre. 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 one per cent, is just too low. But this isotope is radioactive and decays about six times faster than uranium 238 does, 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 three per cent, 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 that 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 physicists confirm 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 USA 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 in which he explained what scientists had surmised about the operation of these ancient reactors.
Cowan described 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.
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 physicists at Washington University in St Louis began to address it by examining a piece of this enigmatic African ore.
Their 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 analyse the isotopic composition of xenon requires a mass spectrometer, an instrument that can separate atoms according to their atomic weight. 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. But to glean greater information about the genesis and retention of this gas, the physicists adopted a more delicate approach called laser extraction, which releases xenon selectively from a single mineral grain, leaving adjacent areas intact.
They applied this technique to many tiny spots on their lone available fragment of Oklo rock, only one millimetre thick and four millimetres across. After each extraction, they purified the resulting gas and passed the xenon into their mass spectrometer, which indicated the number of atoms of each isotope present. Their first surprise was the location of the xenon. It was not, as expected, found to a significant extent in the uranium-rich mineral grains. The lion’s share was trapped in aluminum phosphate minerals that contain no uranium at all. Remarkably, these grains showed the highest concentration of xenon ever found in any natural material. The second surprise was that the extracted gas had a significantly different isotopic make-up from what is usually produced in nuclear reactors. It had seemingly lost a large portion of the xenon 136 and 134 that would certainly have been created from fission, whereas the lighter varieties of the element were modified to a lesser extent.
How could such a change in isotopic composition have come about? Chemical reactions would not do the trick, because all isotopes are chemically identical. Perhaps nuclear reactions, such as neutron capture? Careful analysis allowed the physicists to reject this possibility as well. They also considered the physical sorting of different isotopes that sometimes takes place: heavier atoms move a bit more slowly than their lighter counterparts and can sometimes separate from them. Uranium enrichment plants — industrial facilities that require considerable skill to construct — take advantage of this property to produce reactor fuel. But even if nature could miraculously create a similar process on a microscopic scale, the mix of xenon isotopes in the aluminum phosphate grains would have been different from what the physicists found. For example, measured with respect to the amount of xenon 132 present, the depletion of xenon 136 (being four atomic mass units heavier) would have been twice that of xenon 134 (two atomic mass units heavier) if physical sorting had operated. They did not see that pattern.
Their understanding of the anomalous composition of the xenon came only after they thought harder about how this gas was born. Their key insight was the realisation that different xenon isotopes in their Oklo sample were created at different times — following a schedule that depended on the half-lives of their iodine parents and tellurium grandparents. The longer a particular radioactive precursor lives, the longer xenon formation from it is held off. For example, production of xenon 136 began at Oklo only about a minute after the onset of self-sustained fission. An hour later the next lighter stable isotope, xenon 134, appeared. Then, some days after the start of fission, xenon 132 and 131 came on the scene. Finally, after millions of years, and well after the nuclear chain reactions terminated, xenon 129 formed.
Had the Oklo deposit remained a closed system, the xenon accumulated during operation of its natural reactors would have preserved the normal isotopic composition produced by fission. But scientists have no reason to think that the system was closed. Indeed, there is good cause to suspect the opposite. The evidence comes from a consideration of the simple fact that the Oklo reactors somehow regulated themselves.
How the Oklo reactors probably worked highlights two points: very likely they pulsed on and off in some fashion, and large quantities of water must have been moving through these rocks — enough to wash away some of the xenon precursors, tellurium and iodine, which are water-soluble. The presence of water also helps to explain why most of the xenon now resides in grains of aluminum phosphate rather than in the uranium-rich minerals where fission first created these radioactive precursors. The xenon did not simply migrate from one set of preexisting minerals to another — it is unlikely that aluminum phosphate minerals were present before the Oklo reactors began operating. Instead those grains of aluminum phosphate probably formed in place through the action of the nuclear-heated water, once it had cooled to about 300° Celsius.
During each active period of operation of an Oklo reactor and for some time afterward, while the temperature remained high, much of the xenon gas (including xenon 136 and 134, which were generated relatively quickly) was driven off. When the reactor cooled down, the longer-lived xenon precursors (those that would later spawn xenon 132, 131 and 129, which was found in relative abundance) were preferentially incorporated into growing grains of aluminum phosphate. Which makes one thing clear: the capacity of aluminum phosphate for capturing xenon is truly amazing.
(Original headline: Workings of an ancient nuclear reactor )
