Physicist Francis Perrin was in a nuclear fuel processing plant in the south of France, thinking to himself: "It can't be." The year was 1972. On one side was a dark chunk of naturally occurring radioactive uranium ore, mined in Africa. On the other, accepted scientific data on the constant proportion of radioactive uranium in the ore.
Upon examination of this ore from a mine in Gabon, it was found to contain lower than usual levels of uranium 235 (U 235), the fissile variety. Only slightly lower, but enough for scientists to stop thinking about the unknown.
We want people to learn about natural radioactivity, to be aware that radioactivity is everywhere, that it is natural, and that at low levels it is not dangerous.
Ludovic Ferrière, conservador de la colección de rocas en el Museo de Historia Natural de Viena
The first logical answer that physicists found for such an unusual proportion of U 235 was that it was not natural uranium. Today, all natural uranium contains 0.720% U 235. If we extracted it from the earth's crust, lunar rocks or meteorites, we would find this proportion. But that piece of rock at Oklo contained only 0.717%.
What did that mean? At first, the only option that occurred to the physicists was that the uranium ore had undergone artificial fission, that is, that some of the U 235 isotopes had been caused to split in a nuclear chain reaction. This could explain why the ratio was lower than normal.
But after further analysis, Mr. Perrin and his colleagues confirmed that the uranium ore was completely natural. Even more astonishing was discovering the imprint of fission products in the mineral. Physicists concluded that the uranium ore was natural and had undergone fission. There was only one possible explanation: the rock was proof that natural fission occurred more than two billion years ago.
"After carrying out further studies, including in situ examinations, they found that the uranium ore had undergone fission by itself," explains Ludovic Ferrière, curator of the rock collection at the Natural History Museum in Vienna, where it will be presented. to the public a part of the curious rock in 2019. "There was no other explanation."
For this phenomenon to have occurred naturally, these uranium deposits in western Equatorial Africa had to necessarily contain a critical mass of U 235 to start the reaction. Back then, that's how it was.
A second factor necessary for a nuclear chain reaction to occur and be sustained is the existence of a moderator. In this case, the water. Without water to slow down the neutrons, controlled fission would not have been possible, since the atoms simply would not have split.
"In the same way that if in an artificial light-water nuclear reactor there is nothing to slow down the neutrons, to moderate them, the fission reactions simply stop," says Peter Woods, head of the group in charge of uranium production at the IAEA. "The water acted as a moderator in Oklo, absorbing the neutrons and controlling the chain reaction."
The specific geological context in what is now Gabon also helped. The chemical concentrations of total uranium (including U 235) were high enough and its deposits were thick and large enough. Ultimately, Oklo was able to survive the test of time. Experts suspect that other natural reactors may have existed in the world, but have been destroyed by geological processes, have died out or subducted, or simply have not yet been discovered.
“What makes it so fascinating is that temporal, geological and hydric circumstances have combined to make this happen,” says Mr Woods. “And that it has been preserved until today. The detective story has been successfully resolved.”
A rock sample at the IAEA headquarters city
Rock samples from Oklo, some of which were recovered in drilling campaigns, are stored at the headquarters of French nuclear and renewable energy company Orano. At the beginning of 2018, two halved samples from the drilling were donated to the Natural History Museum in Vienna. The donation was made possible thanks to the financial contribution of Orano and the Commission for Atomic Energy and Alternative Energies (CEA) of France, with support from the Permanent Mission of France to the United Nations and international organizations based in Vienna. IAEA scientists provided support during the shipment of the sample to Vienna by monitoring the levels of radioactivity and enabling safe handling of the rock.
The two samples emit radiation of approximately 40 microsieverts per hour at a distance of 5 centimeters, which is roughly equivalent to the amount of cosmic radiation a passenger would receive on an eight-hour flight from Vienna to New York. The museum, which receives 750,000 visitors a year, is used to handling radioactive samples, as it already exhibits a number of slightly radioactive rocks and minerals.
“We want people to learn about natural radioactivity, to be aware that radioactivity is everywhere, that it is natural, and that at low levels it is not dangerous. There is radioactivity on the floors and walls of our homes, as well as in the food we eat, the air we breathe and even in our own bodies,” says Mr. Ferrière. “What better way to explain it than by exhibiting a real sample from Oklo, where nuclear fission occurred naturally billions of years ago?”
Different sources of background radioactivity will be displayed in the permanent exhibition. Perhaps a world map showing the distribution of radioactivity, a radiation detector or Geiger counter, or a cloud chamber will allow visitors to observe for themselves natural radiation exposure.
“Rocks are like books. On the cover there is basic information, but when you open them there is the whole story”.
