Traditionally, large civil engineering works such as bridges, railways, roads ... have been built, considering the time as a variable to despise, or at least to consider only very slightly. Construction techniques advance, and when do these works are made and make that can withstand thinking enough to be profitable until the new means of construction allow to do better. What is this? At most a thousand years. Undoubtedly, that no sane engineer, project manager for construction of a highway, there is the fact that in 25000 years may enter into an ice age and glaciers will this highway.
However, in the twentieth century came a new type of engineering challenge, and with it, a problem which until then had not been raised. With the advent of the atomic age comes a new form of energy. And as ballast to this development, radioactive waste. Thus born calls for deep geological disposal facilities (AGP's).
It was convinced that the most effective and less dangerous to store this waste was its location in deep geological formations.
However, as engineering works are, the AGP's pose a problem: they have to play their role during lengthy periods of time. We no longer speak of centuries, or a thousand years. Otherwise, the tens and hundreds of thousands of years, because we must not forget that they are high-level waste. For this reason, it requires a knowledge of the behavior of these systems over long periods of time.
For the construction of these storage systems Deep, represents an important part of geochemical modeling, ... geology, field observations and experimental studies and laboratory tests.
However, this process of analysis and study, there are always uncertainties, gaps ... that can not be resolved.
Fortunately, one of those rare glimpses of nature (or perhaps because the Earth is more good than is commonly thought, despite the lack of respect that we process), and cases have existed natural phenomena that have operated for scales considerable time, which are similar engineering the problem before us, and whose study can provide a precious, valuable and unique information. Such cases are known as natural analogues . Let
a paradigmatic case.
OKLO CASE: A CLASSIC AND
begin by describing succinctly what a nuclear power plant.
The most important part of what constitutes a nuclear power reactor, the reactor consists of the following components:
- The fuel : Normally Uranium. Is responsible for producing energy by nuclear fisiíon process, which is the "break" of atomic nuclei into lighter ones. In this process off neutrons, subatomic particles moving at high speed, and are used to "break" other uranium atoms (in the new "breaks" are "shed" new neutrons that are used to "break" other atoms, ... and so on in a process known as chain reaction).
- The moderator : The probability of fission processes take place continuously, and may well take place chain reaction, increases with decreasing neutron velocity. Thus, nuclear reactors incorporate a system consisting of a particular chemical compound, such as graphite rods or other light or heavy water (water whose molecules have a hydrogen isotope known as deuterium), whose mission is to "stop" to neutrons. The use of the moderator, makes talk of nuclear reactors slow neutrons (there is another way, the nuclear reactor fast neutrons, which instead of using a moderator, the sample is enriched in nuclei capable of undergoing fission).
- A driver : bars usually consist of a neutron absorbing material such as boron or cadmium, and to control the development of nuclear reaction by acting on the "number" of neutrons. If neutrons exceeds a critical number, the chain reaction would take place in an uncontrolled manner, energy release would be huge, and inexorably, the nuclear reactor explosionaría. Of this element depends, therefore, nuclear reactor safety.
- A refrigerant : The manager of extracting the energy released as heat during fission, carbon dioxide can be pressurized heavy water, ordinary water or liquid sodium.
Now we consider some fuel, uranium. Uranium in its natural state appears as 3 isotopes, of which here we will consider two, Uranium-238 (the most abundant isotope), and Uranium-235. The isotope 235 is an isotope that says "very fissile" when subjected to the action of neutrons, it "breaks" (really, you should say fissions) into 2 lighter nuclei, while emitting 2.5 neutrons (on average). The isotope 238, by contrast, absorbs many neutrons without undergoing fission process. For this reason, a mass of uranium that had only 238 (or too little of 235, as in natural uranium), once initiated the fission reaction, this would end up stopping for a short time alone, since neutrons (necessary as we have seen, to take place the chain reaction) produced by fission would not be enough to offset the losses by absorption and leaks in the system (some neutrons rebels, escape). This explains the relationship Uranium 238 235/Uranio be called enrichment, and a mass of uranium that has a high percentage of uranium-235 is said to be enriched .
In nature, the enrichment is 0.72% (0.72 atoms of the isotope is 235 per 100 of 238, or whatever it is, there are approximately 2 cores of 235 per 300, 238), and this value is insufficient for the fission reaction from spreading. (Later we shall see, as this percentage has remained, much less constant over geological time.)
Because of this, in nuclear power plants it is necessary to enrich uranium-235 fuel to values, which in the case of slow neutron nuclear plants reach 3% (and the fast neutron can reach 15%). The atomic bombs, meanwhile, use masses of almost pure Uranium 235, resulting in massive release of energy (HEU).
One issue that is important to understand then the case is as follows Oklo How is Uranium in its natural state?
Uranium has a high affinity for oxygen and ions dissolved in water to form complex salts very soluble in water, which gives it great mobility from a geochemical point of view (which already poses a first problem for AGP's, because in these facilities is important is that their mobility is reduced). However, under reducing conditions these compounds are reduced, the compound appeared smaller and more important (in terms of abundance and issues of extraction) of uranium, uraninite or pitchblende (uranium dioxide, UO2). And this compound is insoluble. It is important to retain this chemical property (the insoluble, of course) it will be important from a geological perspective in understanding the dynamics of formation of uranium deposits.
It's time to get down to bedrock. Many opponents of nuclear energy, would be surprised if you read the following statement: the first nuclear history, ran for 2000 million years. No, not talking about ancient astronauts, lost civilizations or aliens who visited Earth in the Precambrian. I'll try to explain.
enriqucimiento Returning to the theme. We have said that their natural state value is 0.72%. However, due to their respective peculiarities, Uranium 235 is fissionable uranium faster than 238. As a result, the value of enrichment have gone down with the course of geological time. So at the time of formation Earth 4500 million years ago, its value was 22%, and 2000 million years ago (which, as we shall see, is the date that we will interest) was 3.6%.
On this basis, in the 50s, the British and Inghram Wetherill one hand, and secondly the U.S. Kuroda, suggested the possibility of making 1000 to 2,000,000,000 years in the uranium deposits may have been given the natural conditions that had been effected the process of nuclear fission. To do so would have fulfilled the condition that these deposits, the ore (a mineral deposit, sterile materials accompanying the useful mineral) was not made by absorbing elements neutrons, such as boron.
In 1972, Naudet and Hageman, two scientists from the Atomic Energy Commission of France, who were working in Africa, they found that the mining company product extracted from a mine Franceville, Gabon, Oklo mine had content too low in uranium-235 (0.3%). After an exhaustive study of this extraordinary discovery, it was convinced that 2000 million years ago, the geological system was functioning as a nuclear reactor.
erosion by streams from the surrounding areas, water enriched uranium. When these fluids were found with conditions reductive uranium complexes were reduced to UO2, insoluble, precipitating and forming uranium deposit. This took place on a granite base, a craton, which are very stable geológicamene areas. In this context, moreover, the gang consisted mainly of O, Si, Al, K, Fe and Mg, low neutron-absorbing elements and also the water that ran on the system could play the role of moderator. We, therefore, as 2000 million years ago, in this case met the conditions for the reaction to take place in nuclear fission chain: a substance, the water, which acted as moderator, and because of its properties something absorbers, also played the role of controller, the absence of super absorbent chemicals into the bargain, and the fact that develops in a stable geological area allowed both uranium and the reaction products are not being prosecuted would see major mobility enables the quantity of which are kept in place for training, even to this day (that after 2000 million years!).
The comprehensive field study revealed that a total of 16 outbreaks in which it reached the critical mass of pitchblende (the minimum mass needed to have enough content to take place neutron chain reaction) held in the fission reaction, and that the system was functioning as a reactor for about 500,000 years.
such low values \u200b\u200bwere explained as Uranium 235 because they were drawn by the chain reaction. In support of this idea is the fact that there were similar percentages (by the consideration of elapsed time) to nuclear power plants, very strange compounds in their natural state, and isotope ratios (such as the Neodymium) rare in natural conditions .
It has been reported, although it has not been given much attention, which has probably been many more systems like this, especially if we consider that older than 2500 million years, the general conditions of Earth's atmosphere was reducing (explaining that the vast majority of pitchblende deposits have that old) and enrichment values \u200b\u200bwere more than significant.
Oklo's case is, therefore, as a benchmark when selecting and studying a site in which to house high-level products, for which it is very important that mobility is reduced. Cases like this, or the Cygar Lake (Canada), where a clay coating prevents the uranium reaches surface water (despite being one of the richest uranium deposits in the world) or the similar case Morro do Ferro (Brazil), are excellent examples of natural analogues in which responses to problems that arise when installing AGP's.
The Oklo event in its uniqueness, has received attention from physicists, because it is trying to find in it the answer to the question of whether physical constants like the speed of light, have remained constant Throughout the history of the universe.
I hope this has proved attractive presentation of the amazing world of natural analogues. To another.
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