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The 'Oklo phenomenon' or how nature creates its own nuclear reactors


Although we think of radioactivity as a kind of 'ethereal poison' that occurs in specific places such as X-ray rooms or nuclear power plants, the truth is that it is something that has been with us since the beginning: 4,500 million years ago. years ago, when our planet was formed, this 'invisible force' was present. And now it is so common that we even eat and breathe it daily. « You yourself are radioactive », affirms the chemist and nuclear engineer Alfonso Martínez , author of the book ' That was not in my radioactivity book' (Guadalmazán, 2020), and that tells all the secrets of this natural physical phenomenon that has the property of impressing photographic plates, ionizing gases, producing fluorescence or passing through opaque bodies, such as shoes, in ordinary light.


Top Image: Oklo samples donated to the Vienna Natural History Museum - Ludovic Ferrière/ Natural History Museum


“When they talk to us about radioactivity, we think of Marie Curie , uranium and radium; but it also comes to mind how dangerous it can be, "says Martínez, who currently works at the Center for Environmental and Technological Energy Research (Ciemat), referring to disasters such as Chernobyl or, more recently, such as Fukushima. However, in his work he shows the daily life of these elements that today "have a reputation for being malignant and dangerous", although at one time they were considered the cure for all ills, prescribed as medicine. However, there are more unknown chapters about how this radioactivity arises even from where it is not expected, and there are places on our planet that can become, meeting certain requirements, natural and spontaneous nuclear reactors , which without the help of man create and they undergo fission chain reactions.


The story begins with the bewilderment of physicist Francis Perrinwhen, one day in 1972, he discovered that a sample of ore taken from a mine in Africa contained a lower proportion of uranium 235 (U 235), the fissile variety, than usual. The analyzes through the mass spectrometer - a tremendously effective tool to measure radioactivity very precisely - indicated 0.717%, when the normal would have been 0.720%. Because, today, all natural uranium contains 0.720% of U 235. If we extracted it from the earth's crust, from rocks from the Moon or from meteorites, we would find this proportion. "That difference was very significant," says Martínez, who explains that this small variation was only understood by a fission process, such as the one that is triggered artificially in nuclear power plants. But,


“Fission is a process by which the nucleus of an atom is broken into two pieces by the impact of a neutron, generating two smaller atoms, a couple more neutrons and a lot of energy. But, in addition, these new neutrons that are generated can produce new fissions, which can cause a chain reaction and thus many more uranium atoms fission”, indicates the author. “The uranium on Earth has decayed according to its half-life and that is why the percentage contained in the samples is always the same, 0.720% of the isotope U 235 with respect to the total uranium. For that amount to be lower something else besides radioactive decay has had to happen for it to go away. And that was the spontaneous fission reactions produced by neutrons coming from the Sun,


And for this to happen, several requirements had to be met: the first is that these uranium deposits in Western Equatorial Africa had to necessarily contain a critical mass of U 235 to start the reaction. And back then, billions of years ago, it was. On the other hand, for the nuclear chain reaction to occur and be maintained, a moderator had to exist. Here it was the water that acted as a neutron retarder. Because, just as if there is nothing in an artificial light-water nuclear reactor to 'moderate' the neutrons, the fission reactions simply stop. Water acted as a moderator in Oklo, absorbing the neutrons and controlling the chain reaction.


Geological situation in Gabon triggering nuclear fission reactors. 1. Zone where fission reactions occurred 2. Sandstone 3. Uranium ore 4. Granite - Wikicommons


In addition, during the operation of the natural nuclear reactor, the typical fission elements were formed, which today have mostly disappeared. "But the stable ones, which of course are still in the deposit, have been analyzed. For example, the fissions that produced several stable isotopes of the element neodymium, whose composition is different from that of other natural samples", indicates Martínez. "On the other hand, the fact that these fission products are not dispersed throughout the deposit tells us that the reactors worked in a controlled manner."


That is, radioactivity is part of life. Although the single word scares more than one.

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