In 1972, a chunk of uranium was extracted from a mine in Africa and discovered to be, in fact, a two billion-year-old natural nuclear reactor called “Oklo” after the place where it was found. Its existence mystified scientists at the time, but research over the decades has been able to explain its unusual properties and how it came to be. Some of the Oklo fragments are housed in Vienna’s Natural History Museum in Austria and serve to illustrate the natural presence of radioactivity in nature.
The discovery of natural fissile uranium ore in Africa in 1972 had scientists stumped
French physicist Francis Perrin was the first scientist to analyze the high-grade uranium ore brought to the surface from a mine in Gabon on the Atlantic coast of Central Africa, and he was flummoxed to find that it contained a lower proportion of uranium-235 (U-235) than expected.
All natural uranium mined today contains 0.720% of U-235 across the board. But the Oklo ore contained only 0.717%.
The fissile radioactive material went against all accepted scientific data about the constant ratio of radioactive uranium in ore and scientists surmised at first that it must have been treated by non-natural means before arriving in Perrin’s hands, suspecting that some of the U-235 isotopes had been forced to split in a nuclear chain reaction.
The scientific community had to accept that natural fission had occurred
Although the scientists suspected artificial fission, they had to later accept that the ore had undergone a process of natural fission in the Earth’s crust more than two billion years before, and fission product traces were left behind for scientists to identify.
Ludovic Ferrière, Curator of the Rock Collection, Natural History Museum, Vienna, Austria, explained:
“After more studies, including on-site examinations, they discovered that the uranium ore had gone through fission on its own. There was no other explanation.”
Fragments of Oklo are on show in Austria
Pieces of the Oklo ore are on display in Vienna’s Natural History Museum. Material accompanying the exhibition educates visitors about the natural presence of radioactivity and illustrates the process of natural nuclear fission. It’s now accepted that low levels of the kind of radiation that’s found in ordinary environments are safe.
Ferrière continued:
“We want people to learn about natural radioactivity, to make them aware of the fact that radioactivity is all around us, that it’s natural, and that at low levels it’s not dangerous.”
How does the natural phenomenon of nuclear fission occur?
When looking at the processes that led to the creation of naturally fissile uranium, it’s assumed that the deposits in Africa must have had to contain a critical mass of U-235 to initiate the reaction. A second essential factor was that there needed to be a moderator for a nuclear chain reaction to occur and be sustained. In this case, the moderator was water, which slowed the neutrons down. Fission would not have been possible without water as the atoms would simply not have split.
The precise conditions 2 billion years ago may have occurred elsewhere
Peter Woods, team leader at the International Atomic Energy Agency (IAEA), expanded:
“Like in a man-made light-water nuclear reactor, the fission reactions, without anything to slow down the neutrons, to moderate them, simply stop. The water acted in Oklo as a moderator, absorbing the neutrons, controlling the chain reaction.”
The specific geological context of the area where Oklo was uncovered played a major role in the formation of the uranium ore.
Experts believe that other natural reactors must have occurred in other parts of the world, but these are likely to have been destroyed by geological processes, eroded, or subducted, or simply not yet found. The possibility of such deposits waiting for discovery and research is an exciting prospect.