by Alex P. Meshik
January 26, 2009
from ScientificAmerican Website
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:
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:
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 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.
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
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.
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.
Although almost all this material, which has a 24,000-year half-life, 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.
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.
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
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.
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.
Of course, we first needed to decide where
exactly to aim the laser beam. Here Hohenberg and I relied on our
colleague Olga Pravdivtseva, who had constructed a detailed x-ray
map of our sample and identified the constituent minerals. After
each extraction, we purified the resulting gas and passed the xenon
into Hohenberg’s mass spectrometer, which indicated the number of
atoms of each isotope present.
The second epiphany was that the extracted gas
had a significantly different isotopic makeup 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.
We 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 thus 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 we studied would have been different from what we found.
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. We did not see that pattern.
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.
after millions of years, and well after the nuclear chain reactions
terminated, xenon 129 formed.
The evidence comes from a consideration of the simple fact that the Oklo reactors somehow regulated themselves. The most likely mechanism involves the action of groundwater, which presumably boiled away after the temperature reached some critical level. Without water present to act as a neutron moderator, nuclear chain reactions would have temporarily ceased.
Only after things cooled off and sufficient groundwater once
again permeated the zone of reaction could fission resume.
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 degrees
When the reactor cooled down, the longer-lived xenon precursors (those that would later spawn xenon 132, 131 and 129, which we found in relative abundance) were preferentially incorporated into growing grains of aluminum phosphate. Then, as more water returned to the reaction zone, neutrons became properly moderated and fission once again resumed, allowing the cycle of heating and cooling to repeat.
The result was
the peculiar segregation of xenon isotopes we uncovered.
The details remain
fuzzy, but whatever the final answers are, one thing is clear: the
capacity of aluminum phosphate for capturing xenon is truly amazing.
This exercise revealed much about the timing of reactor operation, with all xenon isotopes providing pretty much the same answer. The Oklo reactor we studied had switched “on” for 30 minutes and “off” for at least 2.5 hours. The pattern is not unlike what one sees in some geysers, which slowly heat up, boil off their supply of groundwater in a spectacular display, refill, and repeat the cycle, day in and day out, year after year.
This similarity supports the notion not only that groundwater passing through the Oklo deposit was a neutron moderator but also that its boiling away at times accounted for the self-regulation that protected these natural reactors from destruction.
In this regard, it was extremely
effective, allowing not a single meltdown or explosion during
hundreds of thousands of years.
They have also scrutinized a similar zone of ancient nuclear fission found in exploratory boreholes drilled at a site called Bangombe, located some 35 kilometers away.
reactor is of special interest because it was more shallowly buried
than those unearthed at the Oklo and Okelobondo mines and thus has
had more water moving through it in recent times. In all, the
observations boost confidence that many kinds of dangerous nuclear
waste can be successfully sequestered underground.
Nature’s fission reactors suggest the
possibility of locking those waste products away in aluminum
phosphate minerals, which have a unique ability to capture and
retain such gases for billions of years.
For three decades, the two-billion-year old Oklo phenomenon has been used to argue against - having changed.
But last year Steven K. Lamoreaux and Justin R. Torgerson of Los Alamos National Laboratory drew on Oklo to posit that this “constant” has, in fact, varied significantly (and, strangely enough, in the opposite sense from what others have recently proposed).
Lamoreaux and Torger son’s calculations hinge on
certain details about how Oklo operated, and in that respect the
work my colleagues and I have done might help elucidate this
I expect that a
few telltale wisps of xenon could aid immensely in this search.