Earth Made Its Own Nuclear Reactor—2 Billion Years Ago

About 2 billion years ago—long before the dinosaurs but not so long after the first microbes added a huge amount of oxygen to Earth's air—several underground nuclear reactors were sputtering away deep within a seam of rich uranium ore.

Because of continental drift, the region later became West Africa, although it was then close to what's now Brazil. The resulting traces of the natural nuclear reactors near the modern town of Oklo in Gabon are the only ones ever found. Studying them has shown that the laws of physics have remained constant over vast time—and has provided clues for safely storing nuclear waste.

The first sign of the extremely ancient reactors appeared in 1972, when a nuclear technician in France noticed very slight abnormalities in uranium ore from a mine near Oklo. "The technician was monitoring the radioactivity, and he saw this was different," says North Carolina State University nuclear physicist Chris Gould, who made a detailed analysis.

The difference was minuscule—the Oklo ores contained a fraction of a percent less than expected of the radioactive isotope uranium-235 compared to all uranium ores from everywhere else in the world. Some of the radioactive ore had effectively been "burned off" inside a nuclear reactor. And since the Oklo ores were about 2 billion years old, any nuclear reactors there must have occurred naturally. "Here's a tip of the hat to that guy," Gould says, noting that the French technician decided to investigate the tiny discrepancy rather than simply ignoring it.

Scientists think there were more than a dozen natural nuclear reactors underground at what's now the open-pit Oklo mine. The ancient reactor sites lie within a rich vein of uranium near the surface of a landscape of rocky outcrops, savannah and forest. But 2 billion years ago, they were several thousand feet underground, where they were exposed to seeping groundwater. The groundwater acted as a "moderator," effectively enabling the rich uranium ores to more easily split and release energy in a nuclear fission chain reaction.

Gould says the natural mechanism was "comparable to a pressurized-water reactor today."

Nuclear chemist Alex Meshik of Washington University in St. Louis has studied the traces of the Oklo reactors. He explains that four factors were precisely aligned at Oklo to create the natural reactors: the extraordinary richness of the uranium ore vein at the site, groundwater that enabled the fission chain reactions, the unique geochemistry of the underground sites that prevented the reactors from overheating and blowing themselves apart, and the particular geological age when the Oklo reactors operated.

"The most critical is the age of the uranium deposits," he says, noting that they formed after the "Great Oxidation Event" (GOE) about 2.4 billion years ago. At that time, Earth's atmosphere was transformed by oxygen from trillions upon trillions of cyanobacteria that grew in mats in the primeval shallows. Oxygen from this event then formed water-soluble uranium ores at Oklo, and those ores enabled the natural reactors to function, Meshik says.

The reactors at Oklo worked in self-sustaining pulses that lasted from hours to months. Groundwater would seep into the uranium deposits and start a fission reaction. As the reaction heated the ground, the water boiled away and the reaction stopped. When the site cooled, new water flowed in and the reaction began again.

Scientists have identified up to 16 underground reactor sites over about a square mile. The largest produced roughly the power of a car engine. These natural reactors appear to have switched on and off at different times over about 200,000 years, with one shutting down as another started nearby.

Gould wanted to know whether the laws of physics that control nuclear fission today were any different when the Oklo reactors were active 2 billion years ago. The researchers found no change, and their results match what astronomers have observed when they study the distant universe.

When the Oklo reactors were operating, no plants or animals lived on land, but their radiation stayed trapped underground. The reactors produced highly radioactive elements like cesium, strontium and plutonium, yet layers of rock and clay kept the radiation within a few feet of each site. Because these byproducts stayed contained for 2 billion years, the Oklo sites offer lessons for how deep geologic repositories, such as the stalled Yucca Mountain project to store nuclear waste in Nevada, could potentially work. As Gould says, “The waste just sat there for 2 billion years.”