Oklo nuclear reactors, an amazing story in evolution of Earth
Background
In 1905 Albert Einstein published his special theory of relativity. According to this theory, the relation between mass and energy is expressed by the equation E = mc2, it means that a very small amount of mass is equivalent to a vast amount of energy. In 1938 Otto Hahn the German chemist and Fritz Strassmann split the Uranium atom into 2 roughly equal parts by bombardment with neutrons. As the result of these experiments, the Austrian physicist Lise Meitner and Otto Robert Frisch the British physicist, went to explain the process of nuclear fission in 1939, placing the release of nuclear energy within reach.
The release of nuclear energy on a large scale used primarily in military applications. The first atomic bomb was tested on 16th, July 1945 at Alamogordo, New Mexico by the Manhattan Project. On 6th, Aug 1945 the first atomic bomb was exploded by the US over Hiroshima, Japan, and 3 days later the second atomic bomb was dropped on Nagasaki. After 1950 a lot of nuclear reactors were designed to produce electric power, thus the Atomic era was started. But amazingly, Mother Nature had designed the first nuclear reactors about 2 billion years ago, and it seems, knows how to operate these nuclear reactors. In fact a nuclear chain reaction very much like the one that Enrico Fermi and his colleagues famously demonstrated in 1942, had certainly taken place, 2 billion years ago. These nuclear reactors have been survived for 2 billion years of geologic time. This extraordinary event occurred at a place now called Oklo in Gabon, Africa, and was discovered quite by accident. The Oklo reactors ran gently at the kilowatt power level for millions of years and used up a fair amount of the natural U-235 in doing so. It is amazing to find that, in the Proterozoic, the evolution of the essence of the universe built a set of nuclear reactors that ran for millions of years.
Discovery and development of Oklo story
At Oklo, there was a mine that supplied uranium mainly for the French nuclear industry. During the 1970s, a shipment of uranium from Oklo was found to be depleted in the fissionable isotope U-235, while the natural uranium has isotopic composition:
• 99.27% of U-238
• 0.72% of U-235
• Traces of U-234
The most useful uranium isotope for nuclear power is U-235, which today accounts for 0.72% of any given natural sample of uranium. When the solar system first formed, that number would have been more like 17%, falling steadily until it reached the nowadays level. And 2 billion years ago? Scientists estimate Oklo reactors would have had samples with roughly 3.6% Uranium-235 that’s close to the enrichment threshold of today’s nuclear reactors.
However, further investigations into the Oklo deposit discovered uranium ore with a U-235 concentration as low as 0.44%.This loss in U-235 is exactly what happens in a nuclear reactor. A possible explanation, therefore, was that the uranium ore had operated as a natural fission reactor. Other observations led to the same conclusion, and on Sept 25, 1972, the French Atomic Energy Commission announced their finding that self-sustaining nuclear chain reactions had occurred on earth about 2 billion years ago in Oklo deposit. Geochemical evidence shows that the Oklo uranium deposit behaved as a natural nuclear fission reactor in Precambrian times.
Mechanism of the operation of Oklo reactors:
In chemical point of view, Uranium is such that it is insoluble in water under oxygen-free conditions but readily soluble in water in the presence of oxygen. When enough oxygen appeared in the Proterozoic to render the groundwater oxidizing, Uranium in the rocks began to dissolve and, as the UO2 ion, became one of the many elements present in trace quantities in flowing streams. In Oklo deposit such a stream flowed into an algal mat that included microorganisms with a strange capacity to collect and concentrate uranium specifically, and they performed their unconscious task so well that eventually enough uranium oxide was deposited in the pure state for a nuclear reaction to start. When more than a "critical mass" of uranium containing the fissionable isotope is gathered together in one place there is a self-sustaining nuclear chain reaction.
The fission of uranium atoms sets free neutrons that cause the fission of more uranium atoms and more neutrons and so on. Also of the neutrons released during the fission of U-235, were captured by the more abundant uranium 238, which became uranium 239 and, after emitting 2 β-particle, turned into Plutonium-239. More than two tons of this plutonium isotope were generated within the Oklo deposit. Although almost all this material, which has a 24,000-years half-life, has since disappeared (primarily through natural radioactive decay), some of the Plutonium itself underwent fission, as attested by the presence of its characteristic fission products. A nuclear fission reaction is a perverse kind of fire; it burns better when well watered. As mentioned earlier, the isotopic composition of the natural Uranium is 99.27% of U-238 and 0.72% of U-235, but the isotope U-235 decays more rapidly than the common isotope 238U. At beginning the proportion of fissile U-235 was not 0.72%, as now, but was about 33%.
Mechanism of operation of Oklo reactors:
The Oklo natural nuclear reactors formed when a uranium-rich mineral deposit became inundated with groundwater that acted as a neutron moderator, and a nuclear chain reaction took place. The heat generated from the nuclear fission caused the groundwater to boil away, which slowed or stopped the nuclear reaction. After cooling of the mineral deposit, the water returned and the reaction restarted, completing a full cycle every 3-hours. The fission reaction cycles continued for about million of years and ended when the ever decreasing U-235 no longer could sustain a chain reaction.
However in a phenomenal story, 2 billion years ago— eons before humans developed the first commercial nuclear power plants in the 1950s, the Oklo natural nuclear reactors operated in a place in Gabon. The average power output of these reactors was about 100 KW. The low power output of these reactors is not remarkable, but this phenomenon is so remarkable because the Oklo reactors began operating around two billion years ago, and they continued to operate in a stable manner for up to million years. Eventually, the fissionable U-235 was depleted, and Oklo reactors shut down, but the story of Oklo reactors contains deep insights into the universe.
Reference:
• Oklo Reactor, The world's first and only natural nuclear reactor
• Oklo