In 1939, German physicists Otto Hahn and Fritz Strassmann were trying to synthesize chemical elements heavier than uranium when they made an unexpected discovery. By bombarding uranium atoms by neutrons, the scientists sought to agglutinate the resulting particles and produce heavier particles. They verified, however, that the effect obtained was the opposite: after the collisions, the atoms generated several less heavy elements. There, the phenomenon of nuclear fission was discovered, that is, the partitioning of larger atoms into smaller particles, which can occur both naturally and artificially (as when Hahn and Strassman threw neutrons at uranium atoms as if they were billiard balls).
But how can an atom naturally split into several smaller ones? We can think of an atomic nucleus as a soap bubble. If a nucleus has too many protons and neutrons, it can collapse, like a soap bubble that bursts after getting too big, thus going into spontaneous fission.
​The spontaneous and induced fission of uranium atoms generates several lighter elements and gamma rays, as well as releasing other neutrons from the fissioned atomic nucleus. Each uranium fission event produces an average of another 2.5 neutrons, which can be ejected at high speed, hitting other uranium nuclei around. Thus, a chain reaction can be established, as long as there is a sufficient amount of uranium in the material used, the so-called critical mass.
In addition to these particles, each fission event still releases large amounts of energy. And, in 1942, Enrico Fermi, who ended up winning a Nobel Prize in physics, demonstrated that the exploitation of nuclear energy was feasible. In a game room under the bleachers of an abandoned football stadium at the University of Chicago, Fermi built the first nuclear reactor, dubbed the Chicago Pile-1 (CP-1). Capable of controlling the nuclear fission reaction, the device consisted of a pile of uranium tablets separated by blocks of graphite — graphite acts as a moderator, that is, it reduces the speed of neutrons, preventing energy from being released too quickly, as happens in atomic bombs.
Modern nuclear reactors also use other moderators, such as water. The fuel used is uranium artificially enriched in the isotope uranium-235 (isotopes are atoms with the same number of protons, but with different numbers of neutrons; uranium-235 has 143 neutrons), which goes into fission much more easily than others. , such as uranium-238 (which has 146 neutrons). Uranium-235 corresponds to only about 0.7% of all uranium that occurs naturally today, with uranium-238 being the most abundant (98.3%) – without conditions, therefore, to reach critical mass, if it does not receive an extra supplement.
Uranium-235, however, was once much more abundant in the geological past. Its current scarcity is explained by its natural radioactive decay to lead-207, with a half-life (time for half the uranium atoms in a given mass to turn into lead) of about 700 million years. As the Earth is about 4.5 billion years old, it is estimated that natural uranium was once much more naturally enriched in isotope 235 than it is today. Could there have been, in the geological past, a natural concentration of uranium-235 capable of reaching sufficient critical mass to start a chain reaction?
There are indications that it does. In 1972, a mill in France noticed that the ore arriving at the facility was very different than usual. A chemical analysis detected strange enrichments such as rare earths, zirconium and neodymium. Interestingly, the concentration of these elements in the ore was similar to those generated as nuclear waste in modern reactors. This nuclear waste, like the ore in question, is rich in lighter elements that were generated by nuclear fission of uranium in the reactors, in the same way that Hahn and Strassman produced lighter elements from the artificial fission of uranium.
The ore came from the Oklo region of Gabon, and its analysis showed that, somehow, the conditions to generate and maintain a chain reaction were naturally obtained when this ore was formed, about 2 billion years ago. At that time, the composition of natural uranium would reach up to 3% uranium 235 – capable of reaching critical mass if the uranium was concentrated above a certain level. In the case of Oklo, sea water itself acted as a moderator, allowing the total energy generated, estimated at about 15,000 megawatts/year, representing the consumption of six tons of uranium-235, to be maintained at an average of 20 kW for 800 thousand years. That level of energy was enough to cook the rocks around the deposit, leaving its mark on the Earth’s history record. And it would be an interesting natural energy source for the population – if there was any human population at that time…
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FabrÃcio Caxito is a professor of geology, principal researcher in the GeoLife MOBILE project and philosopher at UFMG.
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