A riddle that leads to interesting thoughts!
A nuclear repository is a place where spent nuclear fuel is stored; there are many such places scattered all over the Earth. All of them were built in recent decades to securely hide dangerous by-products activities of nuclear power plants.
But humanity has nothing to do with one of the burial grounds: it is not known who built it and even when - scientists carefully determine its age at 1.8 billion years.
Oklo phenomenon
In 1972, at the developed uranium deposit in Oklo (Africa, Gabon), an inquisitive laboratory assistant noticed that the percentage of U-235 in the ore was below the standard by 0.003%. Despite the apparent insignificance of the deviation, for scientists it was an emergency. In all terrestrial uranium ores and even in samples delivered from the Moon, the uranium content in the ore is always 0.7202%, for what reason was ore containing 0.7171% or even less raised from the mines in Oklo?
Most of all, scientists are frightened by the incomprehensible, therefore, in 1975, in the capital of Gabon, Libreville, a Scientific Conference, where atomic scientists were looking for an explanation for the phenomenon.
After a long debate, they decided to consider the Oklo field the only natural nuclear reactor on Earth. The natural reactor, which arose 1.8 billion years ago and burned for 500 thousand years, burned out, the ore is a decay product. Everyone breathed a sigh of relief - there was one less mystery on Earth.
Alternative point of view
But not all conference participants made such a decision. A number of scientists called it far-fetched, not up to scrutiny. They relied on the opinion of the great Enrico Fermi, the creator of the world's first nuclear reactor, who always maintained that chain reaction can only be artificial - too many factors must coincide by chance. Any mathematician will say that the probability of this is so small that it can be uniquely equated to zero.
But if this suddenly happened and the stars, as they say, converged, then a self-controlled nuclear reaction for 500 thousand years ... At a nuclear power plant, several people monitor the operation of the reactor around the clock, constantly changing its operating modes, preventing the reactor from stopping or exploding. The slightest mistake - and get Chernobyl or Fukushima. And in Oklo, for half a million years, everything worked by itself?
Most stable version
Those who disagree with the version of the natural nuclear reactor in the Gabon mine put forward their theory, according to which the Oklo reactor is a creation of the mind. However, the mine in Gabon is less similar to nuclear reactor, built by a high-tech civilization. However, the alternatives do not insist on this. In their opinion, the mine in Gabon was the place of disposal of spent nuclear fuel.
For this purpose, the place was chosen and prepared perfectly: for half a million years from the basalt "sarcophagus" not a single gram radioactive substance did not enter the environment.
The theory that the Oklo mine is a nuclear repository with technical point vision is much more suitable than the version of the "natural reactor". But closing some questions, she asks new ones. After all, if there was a repository with spent nuclear fuel, then there was also a reactor from where these wastes were brought. Where does he go? And where did the civilization that built the burial ground disappear to?
The survey concerning the origin of life on Earth excites scientists long time. There is a huge amount various theories, which supposedly should give an answer to this difficult question. For example, in opposition to the official scientific theory who considers Darwin's idea about the development of species to be the most reasonable and correct, is the religious doctrine of the creation of man from nothing, the highest Being, who is usually called God. Also in recent times more and more scientists are coming to the conclusion that life on our planet originated thanks to alien civilizations who have visited our solar system. And this last assumption did not arise out of the blue. Every year throughout the globe find various artifacts confirming the presence of more advanced creatures on our planet.
Mysterious mine in Africa
Oklo Region Gabonese People's Republic, is one of the largest deposits of uranium ores on our planet. It should be noted that in the mythology of the tribes inhabiting the territory adjacent to the mine, there are a huge number of different legends associated with this rock formation. Most of them can be reduced to the idea that once the gods were looking for some treasure in the rocks that could make them invincible. It should be noted that such myths are found in many peoples of the world. Therefore, it is not strange that before the events of 1972, these strange stories scholars have not paid due attention.
In 1972, an event occurs that forces us to reconsider our attitude to this place and take the legends of the natives seriously. About 45 years ago booty uranium ore the site was supervised by the French government. It was assumed that the deposits of uranium ores amounted to several million tons. However, what was the surprise of scientists when it became known that the mine was half empty.
It was logical to assume that unknown people were able to mine a dangerous isotope, without the permission of the government of the country, as well as curators from Paris. However, traces of holding similar works not found in the mine area. This event caused a wide resonance in the public, because the missing isotope could be used to make huge amount nuclear weapons. Hastily, a special commission was set up to investigate this mysterious incident.
This was followed by more detailed studies of the deposit. During the investigation, it was found that in fact the concentration of a dangerous isotope in this mine is as low as in the fuel of a nuclear reactor, which was already in use.
After a significant number of experiments and studies, it became known that nuclear reactions took place in this place more than a hundred thousand years ago.
In modern science, there are no precedents when uranium could be leveled without artificially starting the process of molecular fragmentation, i.e. without outside help.
The most logical option may seem that thousands of years ago, intelligent beings were able to start the process of crushing uranium nuclei. This is confirmed by the fact that the researchers found spent uranium and its long-term decay products in this deposit.
Is a natural nuclear reactor possible?
Immediately after this unique discovery, disputes arose in various scientific circles about this phenomenon. After only 3 years in the city of Libreville, the capital of the Gabonese state, scientific symposium, which brought together scientists from around the world to put an end to this difficult dispute.
It should be noted that opinions great multitude, even some researchers admitted that at last mankind managed to find evidence of the existence of extraterrestrial intelligence, that this natural phenomenon nothing more than a giant nuclear reactor, which was created and used for their own needs by aliens. Of course, such bold theories did not meet with support in more conservative scientific circles.
Most of the researchers present at this scientific meeting came to the conclusion that the Oklo phenomenon is the only natural nuclear reactor in the world that started naturally around 200,000 - 100,000 BC.
Scientists came to this conclusion through research American physicist- Nuclear engineer Notanel Barclow. Through various Scientific research, he was able to create a model of how the chemical reactions in this place. At the heart of this mine is a thick basalt slab, which began to collect radioactive sand on its surface. As a result of earthquakes in this rather seismically unstable region, a basalt slab with radioactive sand accumulated on it fell several hundred meters underground. Falling underground, the basalt slab did not remain a monolith, in some places it cracked, The groundwater leaked through multiple cracks, and created the conditions for the occurrence of reactions. Considering that the soil in this place is exclusively clay, it turns out that the necessary substances for the reaction turned out to be in the likeness of a natural cocoon, which became the very natural reactor.
Over time, as processes seismic activity earth plates decreased slightly in this region, the process of uranium accumulation in the formed underground lagoons began. According to modern scientists, in some cases, the percentage of uranium in such a lens could reach 40 - 65 percent of total substances. The process of injecting the critical mass gradually increased and only water, as a natural catalyst, did not allow an explosion, but started the process of atomic fission. Thus, the natural rector began to work. Subsequently, some natural disaster caused the uranium isotope to simply burn out, which put an end to everything. natural process fission of uranium. The entire remainder of the substance was leveled as a result of a sharp cessation of fission; perhaps a local nuclear explosion occurred at this place.
According to the latest calculations by researchers, the power of the underground reactor was about 100 kW, and the power of the explosion that stopped the entire well-established process was 10-20 kT.
Nuclear repository?
However, there are other theories regarding this uranium deposit. Many researchers are not inclined to accept the natural nuclear reactor assumption. In their opinion, science is faced with an example of an ancient nuclear burial ground.
Scientists came to this conclusion after it was proved that a nuclear reaction cannot occur in view of any natural anomalies or phenomena. The fission of uranium occurs exclusively in built environment and artificially. Based on this fact, most experts are convinced that Oklo is the first repository of hazardous waste in the history of mankind.
The location of the mine is more like an attempt to bury a spent isotope, and it should be noted that the location for it is almost ideally chosen. Let's say that the sarcophagus with spent uranium was walled up in a basalt slab. Similar technologies are being used modern science for the storage of hazardous waste, only due to natural disasters and the unstable seismic situation in the region, the sarcophagus burst and waste leaked to the surface. Geological exploration took the increased radioactive background at this place for deposits of uranium ores.
The theory seems plausible and has the right to exist, however, based on it, another logical question arises. What civilization was able to create a nuclear reactor more than 100,000 years ago and then try to get rid of the spent materials by storing them deep in the earth?
Perhaps scientists need to pay more attention to the myths and legends of the peoples that originally inhabit given area. It is in deciphering oral folk tradition and lies the answer to the question regarding the mysterious dew capable of using and regenerating nuclear energy. As mentioned above, the natives are sure that the gods once inhabited this place, and their power had no limits.
Some historians, who are trying to consider the history of mankind, rejecting various conservative dogmas, say that our civilization is not the first to master technology and achieve incredible development.
Increasingly, humanity is faced with various mysterious artifacts, which do not fit into the canonical historical concept and make us think that history moves in a spiral. After all, even before our civilization there were powerful peoples who were able to achieve unprecedented strength but then destroyed themselves. It is necessary to try so that the current civilization does not suffer a similar fate.
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Much that nature has offered us is in itself more perfect and easier than that what a person plans to make, so researchers are studying, first of all, what nature offers us.
But in what will be discussed in this article, everything happened exactly the opposite.
December 2, 1942 a team of scientists University of Chicago under the direction of Nobel laureate Enrico Fermi created the first man-made nuclear reactor. This achievement was kept secret during World War II, as part of the so-called "Manhattan Project" to build the atomic bomb.
Fifteen years after the fission reactor was created by man, scientists began to think about the possibility of the existence of a nuclear reactor created by nature itself. The first official publication on the subject was by the Japanese professor Paul Kuroda (1956), who laid down detailed requirements for any plausible natural reactors, if any, in nature.
The scientist described this phenomenon in detail, and its description is still considered the best (classical) in nuclear physics:
- Approximate age range for natural reactor formation
- The required concentration of uranium in it
- The required ratio of uranium isotopes in it is 235 U / 238 U
Despite careful research, Paul Kuroda was unable to find an example of a natural reactor for his model among the uranium ore deposits on the planet.
A small but critical detail that the scientist overlooked is the possibility of water participating as a chain reaction moderator. He also failed to realize that certain ores can be so porous that they retain required amount water to slow down the neutrons and support the reaction.
Scientists argued that only man is capable of creating a nuclear reactor, but nature turned out to be more sophisticated.
A natural nuclear reactor was discovered on June 2, 1972 by the French analyst Boujigues in southeastern Gabon, West Africa, right in the body of a uranium deposit.
And this is how the discovery happened.
During routine spectrometric studies of the 235U/238U isotope ratio in Oklo ore in the laboratory of the French uranium enrichment plant Pierrelatt, a chemist found a slight deviation (0.00717, compared to the norm of 0.00720).
Nature is characterized by the stability of the isotopic composition of various elements. It is the same all over the planet. In nature, of course, isotope decay processes take place, but heavy elements this is not typical, because the difference in their masses is insufficient for these isotopes to fission during any geo chemical processes. But in the Oklo deposit, the isotopic composition of uranium was uncharacteristic. This small difference was enough to keep scientists interested.
Immediately appeared various hypotheses about the reasons strange phenomenon. Some claimed that the deposit was contaminated with spent alien fuel. spacecraft, others considered it a burial place for nuclear waste that we inherited from the ancient highly developed civilizations. However, detailed studies have shown that such an unusual ratio of uranium isotopes was formed naturally.
Here is the simulated history of this "wonder of nature".
The reactor was put into operation about two billion years ago during the Proterozoic. The Proterozoic is generous with discoveries. It was in the Proterozoic that the basic principles for the existence of living matter and the development of life on Earth were developed. The first multicellular organisms and began to develop coastal waters, the amount of free oxygen in the Earth's atmosphere reached 1%, and prerequisites appeared for the rapid flourishing of life, there was a transition from simple division to sexual reproduction.
And now, at such an important time for the Earth, our "nuclear natural phenomenon" appears.
Still, it is surprising that no other similar reactor has been found in the world. True, according to some reports, traces of a similar reactor were found in Australia. This can only be explained by the fact that in the distant Cambrian period Africa and Australia were one. Another fossilized reactor zone has also been discovered in Gabon, but in a different uranium deposit at Bangombe, 35 kilometers southeast of Oklo.
On Earth, uranium deposits of the same age are known, in which, however, nothing similar happened. Here are just the most famous of them: Devil's Hole and Rainier Mays in Nevada, Pena Blanca in Mexico, Box Canyon in Idaho, Kaimakli in Turkey, Chauvet Cave in France, Cigar Lake in Canada and Owens Lake in California.
Apparently, in the Proterozoic in Africa, a number of unique conditions arose that were necessary to start a natural reactor.
What is the mechanism of such an amazing process?
Probably at first in a certain depression, perhaps in a delta ancient river, a layer of sandstone rich in uranium ore was formed, which rested on a strong basalt bed. After another earthquake, common in that era, the basalt foundation of the future reactor sank several kilometers, pulling the uranium vein with it. The vein cracked, groundwater penetrated into the cracks. In this case, uranium readily migrates with water containing a large amount of oxygen, that is, in an oxidizing environment.
Oxygen-saturated water makes its way through the rock mass, leaches uranium out of it, drags it along with it, and gradually consumes the oxygen contained in it for the oxidation of organics and ferrous iron. When the supply of oxygen is exhausted, the chemical environment in earthly depths from oxidative to reductive. The "wandering" of uranium then ends: it is deposited in rocks accumulating over many millennia. Then another cataclysm raised the foundation to modern level. This scheme is followed by many scientists, including those who proposed it.
As soon as the mass and thickness of the layers enriched with uranium reached critical dimensions, a chain reaction arose in them, and the "unit" started working.
A few words should be said about the chain reaction itself, which is the result of complex chemical processes taking place in a "natural reactor". The 235 U nuclei are the easiest to split, which, absorbing a neutron, are divided into two fragments of splitting and emit two or three neutrons. The ejected neutrons can, in turn, be absorbed by other uranium nuclei, causing decay to escalate.
Such a self-sustaining reaction is controllable, which is what the people who created the nuclear fission reactor took advantage of. In it, control is carried out by means of control rods (made from materials that absorb neutrons well, such as cadmium), which are lowered into the "hot zone". In his reactor, Enrico Fermi used just such cadmium plates to regulate nuclear reaction. The reactor in Oklo was not operated by anyone in the usual sense of the term.
The chain reaction is accompanied by the release a large number heat, so it was still unclear why the natural reactors in Gabon did not explode, and the reactions self-regulated.
Now scientists are sure they know the answer. Researchers from the University of Washington believe that the explosions did not happen due to the presence of mountainous water sources. In various man-made reactors, graphite is used as a moderator, necessary to absorb emitted neutrons and maintain a chain reaction, and in Oklo, water played the role of moderator of the reaction. When water entered the natural reactor, it boiled and evaporated, as a result of which the chain reaction stopped for a while. It took about two and a half hours to cool the reactor and accumulate water, and the duration active period was about 30 minutes, according to Nature.
When the rock cooled, water seeped through again and started a nuclear reaction. And so, flashing, then fading, the reactor, whose power was about 25 kW (which is 200 times less than that of the very first nuclear power plant), worked for approximately 500 thousand years.
In Oklo, as in the rest of the Earth and in solar system in general, two billion years ago, the relative abundance of the isotope 235 U in uranium ore was 3,000 per million atoms. At present, the formation of a nuclear reactor on Earth in a natural way is no longer possible, since there is a shortage of 235 U in natural uranium.
There are more whole line conditions, the fulfillment of which is mandatory to start the natural cleavage reaction:
- high total concentration uranium
- Low concentration of neutron absorbers
- High retarder concentration
- Minimum or critical mass to start the cleavage reaction
In addition to the fact that nature launched the very mechanism of a natural reactor, one cannot but worry about the next, perhaps the most "urgent" question for the world ecology: what happened to the waste of a natural nuclear "power plant"?
As a result of work natural reactor about six tons of fission products and 2.5 tons of plutonium were formed. The bulk radioactive waste"buried" within the crystalline structure of the uranite mineral found in the body of the Oklo ores.
Unsuitable sizes ionic radius elements that cannot penetrate the uranite lattice either interpenetrate or leach out.
The Oaklin reactor "told" mankind about how to bury nuclear waste so that this burial site was harmless to environment. There is evidence that at a depth of more than a hundred meters, in the absence of free oxygen, almost all products of nuclear burials did not go beyond the boundaries of ore bodies. Movements of only elements such as iodine or cesium have been recorded. This makes it possible to draw an analogy between natural processes and technological.
The problem of plutonium migration is attracting the closest attention of environmentalists. It is known that plutonium almost completely decays to 235 U, so its constant amount may indicate that there is no excess uranium not only outside the reactor, but also outside the uranite granules, where plutonium was formed during the reactor activity.
Plutonium is a rather alien element for the biosphere, and it occurs in scanty concentrations. Along with some uranium deposits in the ore, where it subsequently decays, some plutonium is formed from uranium by interaction with neutrons. cosmic origin. In small quantities, uranium can occur in nature in various concentrations in completely different natural environments- in granites, phosphorites, apatites, sea water, soil, etc.
AT this moment Oklo is an active uranium deposit. Those ore bodies that are located near the surface are mined by the quarry method, and those that are at depth are mined by mine workings.
Of the seventeen known fossil reactors, nine are completely buried (inaccessible).
Reactor Zone 15 is the only reactor that is accessible through a tunnel in the reactor shaft. The remains of Fossil Reactor 15 are clearly visible as a light gray-yellow colorful rock, which is composed mainly of uranium oxide.
The light colored streaks in the rocks above the reactor are quartz that crystallized from hot underground water sources that circulated during the period of reactor activity and after its extinction.
However, as an alternative assessment of the events of that distant time, one can also mention next opinion associated with the consequences of the operation of a natural reactor. It is assumed that a natural nuclear reactor could lead to numerous mutations of living organisms in that region, the vast majority of which died out as unviable. Some paleoanthropologists believe that it was high radiation that caused unexpected mutations in African human ancestors roaming just nearby and made them people (!).
Korol A.Yu. - student of class 121 SNNYaEiP (Sevastopol national institute nuclear energy and industry.)
Head - Ph.D. , Associate Professor of the Department of YaPPU SNYaEiP Vah I.V., st. Repina 14 sq. fifty
In Oklo (a uranium mine in the state of Gabon, near the equator, West Africa), a natural nuclear reactor operated 1900 million years ago. Six "reactor" zones were identified, in each of which signs of a fission reaction were found. Remains of actinide decays indicate that the reactor has operated in a slow boil mode for hundreds of thousands of years.
In May - June 1972 with ordinary measurements physical parameters batches of natural uranium delivered to the enrichment plant in the French city of Pierrelate from the African Oklo deposit (a uranium mine in Gabon, a state located near the equator in West Africa) it was found that the isotope U - 235 in the incoming natural uranium is less than the standard one. Uranium was found to contain 0.7171% U-235. Normal value for natural uranium 0.7202%
U - 235. In all uranium minerals, in all rocks and natural waters of the Earth, as well as in lunar samples this ratio is satisfied. The Oklo deposit is so far the only case recorded in nature when this constancy was violated. The difference was insignificant - only 0.003%, but nevertheless it attracted the attention of technologists. There was a suspicion that there had been sabotage or theft of fissile material, i.e. U - 235. However, it turned out that the deviation in the content of U-235 was traced all the way to the source of uranium ore. There, some samples showed less than 0.44% U-235. Samples were taken throughout the mine and showed a systematic decrease in U-235 across some veins. These ore veins were over 0.5 meters thick.
The suggestion that U-235 "burned out", as happens in the furnaces of nuclear power plants, at first sounded like a joke, although there were good reasons for this. The calculations showed that if mass fraction groundwater in the reservoir is about 6%, and if natural uranium is enriched to 3% U-235, then under these conditions a natural nuclear reactor can start working.
Since the mine is located in a tropical zone and quite close to the surface, the existence of a sufficient amount of groundwater is very likely. The ratio of uranium isotopes in the ore was unusual. U-235 and U-238 are radioactive isotopes with different half-lives. U-235 has a half-life of 700 million years, and U-238 decays with a half-life of 4.5 billion. The isotopic abundance of U-235 is in nature in the process of slowly changing. For example, 400 million years ago natural uranium should have contained 1% U-235, 1900 million years ago it was 3%, i.e. the required amount for the "criticality" of the vein of uranium ore. It is believed that this was when the Oklo reactor was in a state of operation. Six "reactor" zones were identified, in each of which signs of a fission reaction were found. For example, thorium from the decay of U-236 and bismuth from the decay of U-237 have only been found in the reactor zones in the Oklo field. Residues from the decay of actinides indicate that the reactor has been operating in a slow boiling mode for hundreds of thousands of years. The reactors were self-regulating, since too much power would lead to the complete boiling off of the water and to the shutdown of the reactor.
How did nature manage to create the conditions for a nuclear chain reaction? First, in the delta of the ancient river, a layer of sandstone rich in uranium ore was formed, which rested on a strong basalt bed. After another earthquake, common at that violent time, the basalt foundation of the future reactor sank several kilometers, pulling the uranium vein with it. The vein cracked, groundwater penetrated into the cracks. Then another cataclysm raised the entire "installation" to the current level. In nuclear furnaces of nuclear power plants, fuel is located in compact masses inside the moderator - a heterogeneous reactor. This is what happened in Oklo. Water served as a moderator. Clay "lenses" appeared in the ore, where the concentration of natural uranium increased from the usual 0.5% to 40%. How these compact lumps of uranium were formed is not precisely established. Perhaps they were created by seepage waters that carried away clay and rallied uranium into a single mass. Once the mass and layer thickness, enriched with uranium, reached a critical size, a chain reaction occurred in them, and the installation began to work. As a result of the operation of the reactor, about 6 tons of fission products and 2.5 tons of plutonium were formed. Most of the radioactive waste remains inside the crystalline structure of the uranite mineral, which is found in the body of the Oklo ores. Elements that could not penetrate the uranite lattice due to too large or too small ionic radius diffuse or leach out. During the 1900 million years that have passed since the operation of the reactors in Oklo, at least half of the more than thirty fission products were bound in the ore, despite the abundance of groundwater in this deposit. Associated fission products include the elements: La, Ce, Pr, Nd, Eu, Sm, Gd, Y, Zr, Ru, Rh, Pd, Ni, Ag. Some partial Pb migration was detected and Pu migration was limited to less than 10 meters. Only metals with valency 1 or 2, i.e. those who have high solubility in the water, were carried away. As expected, almost no Pb, Cs, Ba, and Cd remained in place. The isotopes of these elements have relatively short half-lives of tens of years or less, so that they decay to a non-radioactive state before they can migrate far in the soil. Of greatest interest from the point of view of long-term problems of environmental protection are the issues of plutonium migration. This nuclide is effectively bound for almost 2 million years. Since plutonium by now almost completely decays to U-235, its stability is evidenced by the absence of excess U-235 not only outside the reactor zone, but also outside the uranite grains, where plutonium was formed during the operation of the reactor.
This unique nature existed for about 600 thousand years and produced approximately 13,000,000 kW. hour of energy. Its average power is only 25 kW: 200 times less than that of the world's first nuclear power plant, which in 1954 provided electricity to the city of Obninsk near Moscow. But the energy of the natural reactor was not wasted: according to some hypotheses, it was the decay of radioactive elements that supplied the heating Earth with energy.
Perhaps the energy of similar nuclear reactors was added here. How many are hidden underground? And the reactor at that Oklo in that ancient time was certainly no exception. There are hypotheses that the work of such reactors "spurred" the development of living beings on earth, that the origin of life is associated with the influence of radioactivity. The data show more high degree evolution of organic matter as we approach the Oklo reactor. It could well have influenced the frequency of mutations of unicellular organisms that fell into the zone advanced level radiation, which led to the appearance of human ancestors. In any case, life on Earth arose and went a long way of evolution at the level natural background radiation, which has become a necessary element in the development of biological systems.
The creation of a nuclear reactor is an innovation that people are proud of. It turns out its creation has long been recorded in the patents of nature. Having designed a nuclear reactor, a masterpiece of scientific and technical thought, a person, in fact, turned out to be an imitator of nature, which created installations of this kind many millions of years ago.
Natural nuclear reactors exist! At one time, the outstanding atomic physicist Enrico Fermi pathetically declared that only a person could create an atomic reactor ... However, as it turned out many decades later, he was wrong - he also produces nuclear reactors! They existed for many hundreds of millions of years ago, bubbling with nuclear chain reactions. The last of them, the natural nuclear reactor Oklo, went out 1.7 billion years ago, but still breathes radiation.
Why, where, how, and most importantly, what are the consequences of the emergence and activity of this natural phenomenon?
Natural nuclear reactors may well be created by Mother Nature herself - for this it will be enough for the necessary concentration of the uranium-235 isotope (235U) to accumulate in one "place". An isotope is a kind of chemical element, which differs from others by a greater or lesser number of neutrons in the nucleus of an atom, while the number of protons and electrons remains constant.
For example, uranium always has 92 protons and 92 electrons, however, the number of neutrons varies: 238U has 146 neutrons, 235U has 143, 234U has 142, 233U has 141, etc. ... In natural minerals - on Earth, on other planets and in meteorites - the bulk is always 238U (99.2739%), and the isotopes 235U and 234U are represented only by traces - 0.720% and 0.0057%, respectively.
A nuclear chain reaction begins when the concentration of the uranium-235 isotope exceeds 1% and the more intense it is, the more it is. Precisely because the uranium-235 isotope is very scattered in nature, it was believed that natural nuclear reactors could not exist. By the way, in nuclear reactors of power plants, as fuel, and in atomic bombs 235U is used.
However, in 1972 in uranium mines near Oklo, in Gabon, Africa, scientists discovered 16 natural nuclear reactors that were active almost 2 billion years ago ... Now they have already stopped, and the concentration of 235U in them is less than it had to be in "normal" natural conditions — 0,717%.
This, albeit meager, difference, compared with "normal" minerals, led scientists to draw the only logical conclusion - natural atomic reactors really operated here. Moreover, the confirmation was the high concentration of decay products of uranium-235 nuclei, similar to what happens in artificial reactors. When an atom of uranium-235 decays, neutrons escape from its nucleus, hitting the nucleus of uranium-238, they turn it into uranium-239, and that, in turn, loses 2 electrons, becoming plutonium-239...
It was this mechanism that generated more than two tons of plutonium-239 in Oklo. Scientists calculated that at the time of the "launch" of the natural nuclear reactor Oklo, about 2 billion years ago (the half-life of 235U is 6 times faster than 238U - 713 million years), the share of 235U was more than 3%, which is equivalent to industrial enriched uranium.
In order for the nuclear reaction to continue, a necessary factor was the slowing down of fast neutrons that flew out of the nuclei of uranium-235. This factor, as in man-made reactors, was ordinary water.
The reactor began to work at the time of the flooding of uranium-rich porous rocks in Oklo groundwater, and acted as some kind of neutron moderators. The heat released as a result of the reaction caused the water to boil and evaporate, slowing down and subsequently stopping the nuclear chain reaction.
And after the whole rock cooled and all short-lived isotopes decayed (these are the so-called neutron poisons, which are able to absorb neutrons and stop the reaction), water vapor condensed, flooding the rock, and the reaction resumed.
The scientists calculated that the reactor was “turned on” for 30 minutes until the water evaporated, and “turned off” for 2.5 hours until the steam condensed. This cyclical process resembled modern geysers and continued for several hundred thousand years. During the decay of nuclei of uranium decay products, mainly radioactive isotopes iodine, five isotopes of xenon were formed.
It was all 5 isotopes in various concentrations that were found in such rocks of a natural reactor. It was the concentration and ratio of the isotopes of this noble gas (xenon is a very heavy and radioactive gas) that made it possible to establish the frequency with which the Oklo reactor “worked”.
The decay of the nucleus of an atom of uranium-235th ( big atoms) causes the emission of fast neutrons, for a further nuclear reaction must be slowed down by water (small molecules)
It is known that high radiation is detrimental to living organisms. Therefore, in the places of existence of natural nuclear reactors, obviously, there were "dead spots", where there was no life, because DNA is destroyed by radioactive ionizing radiation. But at the edge of the spot, where the radiation level was much lower, there were frequent mutations, which means that new species constantly arose.
Scientists still do not clearly know how life on Earth began. They only know that this required a strong energy impulse, which would have contributed to the formation of the first organic polymers. It is believed that such impulses could be lightning, volcanoes, meteorite and asteroid falls, however, in last years offered for starting point consider the hypothesis that natural nuclear reactors could create such an impulse. Who knows …
