Primordial Nukes

The 2-billion-year-old tale of Earth's natural nuclear reactors

For more than a decade, Alexander P. Meshik has kept close tabs on a fleck of black rock no larger than an infant’s fingernail. It’s so unassuming that most people would sweep it into a dustpan without a second thought. Yet to Meshik, a nuclear physicist originally from Russia, this little scrap of mineral is a scientific gem.

MELLOW YELLOW. Oxidized from exposure to air, the depleted uranium core of one of the Oklo mine’s ancient underground nuclear reactors looks yellow. F. Gauthier-Lafaye. EOST/CGS-CNRS-ULP. France

E. Roell

OUT OF AFRICA. Decades of digging at the Oklo mine exposed remains of many defunct primordial nuclear reactors, including one hidden temporarily by excavators in a metal structure (lower right) to preserve it. Less than a single centimeter across, Meshik’s fleck (inset) of Oklo rock has yielded clues to ancient reactor timing. F. Gauthier-Lafaye. EOST/CGS-CNRS-ULP. France; (Inset) Meshik
PIT STOP. Today, water fills the Oklo mine’s pit, which was permitted to flood, even covering the sites of the reactors, after all of the mine’s uranium ore had been removed. F. Gauthier-Lafaye. EOST/CGS-CNRS-ULP. France

The fleck, with its clues to believe-it-or-not geophysical events, emerged from the bowels of Earth decades ago. It was unearthed in the early 1970s at the Oklo uranium mine in the west African Republic of Gabon. There scientists discovered something startling: mineral evidence of a naturally occurring nuclear fission reactor that had spontaneously sprung to life almost 2 billion years ago and operated for hundreds of thousands of years.

Scientists explain the formation of what has turned out to be 18 reactors in and near Oklo without invoking exotic physics or visitations by space aliens. The sites were fortuitous accumulations of uranium that went critical for a while. Still, many researchers are awed that nature could achieve this result—something that people accomplished only after centuries of scientific and technological advancements.

The ancient reactors are “one of the greatest natural phenomena that ever occurred,” exclaims theoretical physicist Steve K. Lamoreaux of Los Alamos (N.M.) National Laboratory.

Since the Oklo discovery, scientists have bored, charted, measured, and analyzed samples of rock in and around the mine in many ways. But since the end of the 1990s, after the Oklo uranium ores had been completely excavated and the mine closed, only a handful of scientists working on previously obtained samples have persevered in eking secrets from the ancient reactors.

Meshik, now at Washington University in St. Louis, and his colleagues turn to their natural-reactor studies only in their spare time. Their portal to discovery has been that single, unassuming rock chip, made of uranium oxide mingled with minerals, which French researchers gave to Meshik when he was still living in Russia.

Using sophisticated microanalysis techniques, the St. Louis scientists have extracted major new findings about how the natural reactors might have operated. As a practical bonus, Meshik notes, the results might also point toward better ways to immobilize radioactive waste from nuclear-power plants.

In a different vein, but also using data from the Oklo reactor, Lamoreaux and his colleagues are challenging one of theorists’ most basic descriptions of the universe. They present evidence that at least one of the constants of physics can vary over time.

The 0.003-percent solution

In May 1972, the analytical lab at a uranium-enrichment plant in Pierrelatte, France, found the first hint that there had once been a natural reactor at Oklo. The clue was a deficit of just 0.003 percent in the concentration of an isotope—uranium-235—in a gaseous compound produced at the plant. That tiny discrepancy triggered an investigation by France’s nuclear-energy agency of possible contamination in the plant’s processing line.

After tracing the deficit back to ore from the Oklo mine, investigators measured other isotopes in the deposits there and found other telling results. This led them to the dramatic conclusion that a nuclear-fission reactor had spontaneously formed and operated at the Oklo site billions of years before.

In the 1950s, chemist Paul K. Kuroda of the University of Arkansas in Fayetteville had recognized the possibility of such a natural chain reaction. After the discovery of the U-235 deficit and other isotopic clues in Oklo ore, other researchers deduced what combination of geophysical and biological processes could have created the reactor. Nonetheless, many details about how the ancient reactors assembled and operated remain unresolved.

Science fission

“The first question is why [this Oklo reactor] didn’t explode like human-made reactors sometimes do,” Meshik says. “Certainly, it had some sort of self-regulation mechanism.” Modern reactors are controlled by rods made of materials that sop up neutrons generated during fission so that they can no longer trigger additional nuclear decays.

Clues to the Oklo reactor self-regulation arose from studies of the noble gas xenon in the baby nail–size shard that Meshik had obtained years before. When he and his Washington University colleagues Charles M. Hohenberg and Olga V. Pravdivtseva vaporized bits of it with a laser and passed the vapor through a sensitive mass spectrometer, they measured the highest concentrations of xenon ever observed in any natural material. The scientists reported their finding in the Oct. 29, 2004 Physical Review Letters.

Their report “is the most exciting one I’ve read on Oklo for a long time,” comments Etienne Roth, a retired French physical chemist who led early studies of the Oklo deposits.

The St. Louis team hadn’t expected a glut of xenon outside the uranium-rich parts of the sample where xenon is a radioactive breakdown product. But xenon did appear in cavities containing aluminum phosphate, a uranium-free mineral.

“That was a huge surprise to us,” Meshik recalls.

It was also a strong sign, say the researchers, that the fissioning of uranium in at least one part of the Oklo mine operated in a cyclic fashion. Their reasoning goes like this:

Fission of uranium atoms, in which they spontaneously shatter into less-massive atoms, produced precursors of xenon gas. These precursors then migrated into aluminum phosphate as that mineral was crystallizing. When those precursors later decayed into volatile xenon atoms, the cagelike crystal structure of the aluminum phosphate prevented the gas’ escape.

That ancient scenario of gas capture, however, couldn’t take place when the reactors were operating and generating scads of heat. Without significant cooling, aluminum phosphate wouldn’t have crystallized. So, Meshik proposes that the reactor periodically shut down.

More specifics on the cycling come from the relative concentrations of the five xenon isotopes that the researchers examined in their Oklo chip. The data have served as a stopwatch monitoring the timing of the on-off cycles of the reactor.

The Washington University team knew that fission of uranium atoms could directly produce isotopes of xenon. Such buildup of xenon could have occurred only during the reactor’s active periods.

Xenon isotopes also result when fission creates isotopes of other elements, such as tin, antimony, or iodine, that undergo a further series of radioactive decays. This xenon would have accumulated during both active and inactive periods.

Over many millennia of operation, every cycle would have generated the same mix of xenon isotopes. The researchers have used the known rates of all of those xenon-producing processes and the isotope concentrations that they have measured in their sample to calculate that the reactor turned on for about half an hour and then shut down for some two-and-a-half hours before turning on again.

Water works

Water is at the heart of this frequent cycling, says Meshik. Neutrons emitted from splitting atomic nuclei are usually too fast to trigger subsequent fissions, but water has a knack for slowing down neutrons. Therefore, it can provide the braking mechanism required for sustaining chain reactions.

Since the earliest Oklo studies, scientists have proposed a role for water in the creation of the natural reactors. According to the leading theory, deposits of the radioactive metal formed when water running off vegetation-free landmasses carried uranium compounds to the delta of an ancient African river. The theory holds that early biological processes produced oxygen that accumulated in the planet’s atmosphere and oxidized the uranium, making it soluble.

Once those compounds had settled in the delta, geological processes such as uplifting of earth layers and erosion eventually positioned the deposits of concentrated uranium underground, where they were bathed in enough water to start the chain reactions. Under those circumstances, the scientists propose that in each 30-minute active phase, the reactor became so hot that the water boiled away and the reactor stopped working. Then, as the reactor cooled, inflowing water no longer evaporated and so could again slow down neutrons, restarting the cycle.

The current view of the natural underground reactors opens a dramatic prospect. If the reactor was shallowly buried, there might have been a spectacular geyser each time the reactor temperature reached the boiling point of the liquid.

Nothing so dramatic occurs in today’s nuclear reactors built for power generation, although water also serves as a cooling fluid in them. Modern reactors do vent radioisotopes of xenon and other noble gases into the air.

“In the ancient reactor, the aluminum phosphate captured almost all the noble gases, effectively reducing environmental contamination, Meshik notes. “There may be something to learn there in terms of storing nuclear waste.”

Constant worry

Rather than focusing on the workings of ancient nuclear reactors, as Meshik and most other Oklo researchers do, Lamoreaux exploits the Gabon-reactor remnants to test a fundamental assumption about the universe.

Scientists generally assume that the laws of physics, such as Newton’s laws of motion, remain the same throughout space and time, and that a few essential characteristics of the universe are also immutable. Among those supposed universal constants are the speed of light in a vacuum, the mass of the electron, and the strength of the electromagnetic force.

Without that constancy, the universe could be quite different from the description that scientists have deduced so far. The cosmos, for instance, might include unseen dimensions.

Shortly after the Oklo fossil reactor was discovered, researchers recognized that it might provide data that could help them learn about the number called alpha, also known as the fine-structure constant, that’s associated with the strength of the electromagnetic force. Previous studies had always found alpha to be rock steady.

Not so, Lamoreaux and his Los Alamos colleague Justin R. Torgerson reported in the June 15, 2004 Physical Review D. They came to a different conclusion after reexamining a process that was an important sideshow to the chain reactions that kept the Oklo reactor going.

On the main stage, a fissioning uranium atom releases two to three neutrons. If another uranium atom’s nucleus takes in such a neutron, that atom itself fissions, releasing additional neutrons, which can each go on to cause a further uranium atom to fission.

Yet many other atomic nuclei, not just ones that undergo fission, absorb neutrons of certain energies set by the value of alpha. Those sideshow absorptions cause the atoms to change into heavier isotopes or even transmute into other elements. Such transformations also contributed to the specific mix of isotopes that has been found in the ores of the natural reactors.

Lamoreaux and Torgerson focus on two such isotopes—samarium-149 and gadolinium-155—each of which transforms into a heavier isotope upon absorbing a neutron.

Lamoreaux says that researchers in previous studies assumed an unrealistic distribution of neutron energies in the ancient reactors. Those scientists therefore expected many free neutrons that the uranium and water would actually sponge up, he contends.

Recalculating a more likely neutron energy spectrum, he and Torgerson determine that there would have been an excess of neutrons at the right energies for absorption by samarium-149 and gadolinium-155 atoms. That abundance would have yielded more heavier isotopes than have been actually measured.

For the measurements to make sense, the value of alpha had to have been different when the Oklo reactor was operating, the scientists say. The best fit to the isotope data is for alpha to have decreased slightly over the last 2 billion years, they conclude.

Some recent astrophysical observations have also indicated that alpha hasn’t been constant (SN: 10/6/01, p. 222: Constant Changes). Curiously, the evidence from analyses of the radiation emitted by quasars suggests that alpha has been increasing over the eons rather than shrinking. This claim remains controversial (SN: 5/8/04, p. 301: Available to subscribers at Fundamental constant didn’t vary after all).

There’s no contradiction between the quasar and Oklo findings, says John D. Barrow of the University of Cambridge in England of the team that developed the quasar evidence. “One does not expect any variation seen on astronomical scales … to be exactly mirrored in the solar system or inside the Earth,” he says. “The local variation in alpha will reflect local conditions.”

To determine whether alpha has changed at Oklo, Lamoreaux would like to see researchers remeasure the isotopic mixtures in their reactor samples. In this field of isotopic analysis, retesting can be a rewarding approach.

Indeed, the St. Louis team’s recent reactor-cycling evidence resulted from an ability to pinpoint particular isotopes’ concentrations with new precision. “For the last 40 years, the measuring techniques have improved dramatically,” Meshik says.

The current analytic technique, which can discern the compositions of individual mineral grains, requires vaporizing only “a few nanograms of the material,” Meshik notes. Unblemished by those tests, the nondescript fleck of dull black stone that he has kept in his desk all these years remains a gem.

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