Otto Hahn and Fritz Strassmann carried out their first nuclear fission on December 17, 1938, and Lise Meitner and Otto Frisch provided the first physical-theoretical explanation for this experiment a few weeks later. Finally, Enrico Fermi achieved the first self-sustaining nuclear fission chain reaction in Chicago on December 2, 1942.
APA / NHM WIEN / LUDOVIC FERRIERE
"But nature was first there, "says NHM Director General Christian Köberl. The starting point was a sedimentary basin in the present-day Oklo area near the city of Franceville, where uranium has been deposited in a high concentration. Suitable conditions led to a nuclear chain reaction that lasted for hundreds of thousands of years. Köberl had learned from the Oklo reactor a few years after his discovery as a student, "and since then the topic has not let me go."
The natural reactor was discovered in 1972 by a Frenchman. In a French uranium enrichment facility, an anomaly in the isotope ratio of a uranium compound produced from the Oklo ore was noted. In search of explanations, the natural reactor has been discovered and 17 of them are now known in the region.
Today, the fraction of fissile uranium-235 (half-life: 700 million years) is 0.7202 percent throughout the world. Therefore, you have to enrich the uranium to at least three percent uranium-235-share in order to use it as fuel. Two billion years ago, the proportion of the isotope was still at this value, that is, in a concentration where a chain reaction can in principle be used.
But it was long puzzling how the process in nature actually took place. Finally, in every nuclear power plant with great technical effort, the chain reaction must be precisely controlled so that it remains upright, on the other hand does not get out of hand.
Water as moderator
For this one needs absorbers, the correct number at swallowing neutrons released by a nuclear annoyance; on the other hand, a moderator who slows down some neutrons to the right speed in order to be able to split more nuclei – for example, water. Both were obviously present in Oklo, as US researchers discovered in 2004. The water in the porous sandstone layer in which the uranium ore is located may have been a crucial factor in the natural regulation of the process and acted as moderator.
Urged enough water into the uranium ore, could use the chain reaction. As a result, the temperature rose in the rock and the water evaporated. Without this neutron brake, the nuclear reaction collapsed again – until enough water was added.
With this interaction, the natural reactors have been active for thousands of years, setting a thermal power of – compared to the current megawatt nuclear power reactors up to 100 kilowatts free. At some point, there was not enough fissile uranium-235 left to sustain a chain reaction.
The only sample in the museum
Since 2012, chief curator of the NHM rock collection, Ludovic Ferriere, has been trying to obtain a sample from Oklo. a few months ago it finally worked and the first analyzes by the museum are complete. It is a complete, divided into two halves almost ten inches long core of four centimeters in diameter and a total weight of about 320 grams. The fine-grained, almost black sedimentary rocks contain numerous tiny uraninite crystals.
Today, Monday, evening, the sample is handed over to the museum by the Mining Company Orano Mining at a reception at the NHM, sponsoring, for example, the French Atomic Energy Agency CEA. According to Köberl, the NHM is the only museum in the world to have an Oklo probe.
Part of a new exhibition
The natural reactor section is said to be the nucleus of a small permanent NHM exhibition on natural radioactivity. "We try to trace the topic of radioactivity, which is very emotionally charged in Austria, to facts and to show that radioactivity is part of the environment and omnipresent," says Köberl. As an example, he cites the possible high concentration of the radioactive noble gas radon especially in home cellars in the Waldviertel or the fact that "even in our bodies permanently radioactive decays take place."
With the part of the natural reactor you have now a first Object to tell this story. Presumably from 2019, a number of natural radioactive objects will be shown in a showcase of the rock collection. The concept for this is now being worked out and you are trying to set the funding for it. "Perhaps it will be possible to set up a detector that shows the cosmic radiation to which we are permanently exposed," hopes Köberl.
Future visitors would not have to be afraid of "the amount of radioactivity that is emitted by this sample trivial, "says Köberl. According to Ferriere, the radiation exposure of the Oklo sample (both halves together) at a distance of five centimeters is around 40 microsieverts per hour, slightly more than the radiation exposure of a 5,000 kilometer flight (30 microsieverts), much less than a lung x-ray (100 microsievert).
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