Uranium 235 half life how many years. atomic weapons

(β −)
235Np()
239 Pu()

Unlike the other, most common isotope of uranium, 238 U, a self-sustaining nuclear chain reaction is possible in 235 U. Therefore, this isotope is used as a fuel in nuclear reactors, as well as in nuclear weapons.

Formation and decay

Uranium-235 is formed as a result of the following decays:

The decay of uranium-235 occurs in the following ways:

Forced division

About 300 isotopes of various elements were found in the fission products of uranium-235: from =30 (zinc) to Z=64 (gadolinium). The dependence curve of the relative yield of isotopes formed during irradiation of uranium-235 with slow neutrons on the mass number is symmetrical and resembles the letter "M" in shape. The two pronounced maxima of this curve correspond to mass numbers 95 and 134, while the minimum falls within the range of mass numbers from 110 to 125. Thus, the fission of uranium into fragments of equal mass (with mass numbers 115-119) occurs with a lower probability than asymmetric fission. such a tendency is observed in all fissile isotopes and is not associated with any individual properties of nuclei or particles, but is inherent in the very mechanism of nuclear fission. However, the asymmetry decreases as the excitation energy of the fissile nucleus increases, and at a neutron energy of more than 100 MeV, the mass distribution of fission fragments has one maximum corresponding to symmetric fission of the nucleus. The fragments formed during the fission of the uranium nucleus, in turn, are radioactive, and undergo a chain of β - decays, in which additional energy is gradually released over a long time. The average energy released during the decay of one uranium-235 nucleus, taking into account the decay of fragments, is approximately 202.5 MeV = 3.244 10 −11 J, or 19.54 TJ / mol = 83.14 TJ / kg.

Nuclear fission is just one of the many processes that are possible during the interaction of neutrons with nuclei; it is he who underlies the operation of any nuclear reactor.

Nuclear chain reaction

During the decay of one 235 U nucleus, from 1 to 8 (on average - 2.416) free neutrons are usually emitted. Each neutron produced during the decay of the 235 U nucleus, subject to interaction with another 235 U nucleus, can cause a new decay event, this phenomenon is called nuclear fission chain reaction.

Hypothetically, the number of neutrons of the second generation (after the second stage of nuclear decay) can exceed 3² = 9. With each subsequent stage of the fission reaction, the number of neutrons produced can grow like an avalanche. Under real conditions, free neutrons may not generate a new fission event, leaving the sample before the capture of 235 U, or being captured both by the 235 U isotope itself with its transformation into 236 U, and by other materials (for example, 238 U, or by the resulting nuclear fission fragments, such as 149 Sm or 135 Xe).

In real conditions, reaching the critical state of uranium is not so easy, since a number of factors affect the course of the reaction. For example, natural uranium consists of only 0.72% 235 U, 99.2745% is 238 U, which absorbs neutrons produced during the fission of 235 U nuclei. This leads to the fact that in natural uranium at present the fission chain reaction is very fades quickly. There are several main ways to carry out an undamped fission chain reaction:

• Increase the volume of the sample (for uranium extracted from the ore, it is possible to achieve a critical mass due to an increase in volume);
• Carry out isotope separation by increasing the concentration of 235 U in the sample;
• Reduce the loss of free neutrons through the surface of the sample by using various types of reflectors;
• Use a neutron moderator to increase the concentration of thermal neutrons.

Isomers

• Mass excess: 40920.6(1.8) keV
• Excitation energy: 76.5(4) eV
• Half-life: 26 min
• Spin and parity of the nucleus: 1/2 +

The decay of the isomeric state is carried out by isomeric transition to the ground state.

Application

• Uranium-235 is used as fuel for nuclear reactors in which managed fission nuclear chain reaction;
• Highly enriched uranium is used to create nuclear weapons. In this case, to release a large amount of energy (explosion) is used uncontrollable chain nuclear reaction.

see also

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Notes

1. G.Audi, A.H. Wapstra, and C. Thibault (2003). "". Nuclear Physics A 729 : 337-676. DOI:10.1016/j.nuclphysa.2003.11.003 . Bibcode :.
2. G. Audi, O. Bersillon, J. Blachot and A. H. Wapstra (2003). "". Nuclear Physics A 729 : 3–128. DOI:10.1016/j.nuclphysa.2003.11.001 . Bibcode :.
3. Hoffman K.- 2nd ed. erased - L.: Chemistry, 1987. - S. 130. - 232 p. - 50,000 copies.
5. Fialkov Yu. Ya. The use of isotopes in chemistry and the chemical industry. - Kyiv: Tehnika, 1975. - S. 87. - 240 p. - 2,000 copies.
6. . Kaye & Laby Online. .
7. Bartolomey G. G., Baibakov V. D., Alkhutov M. S., Bat G. A. Fundamentals of the theory and methods for calculating nuclear power reactors. - M .: Energoatomizdat, 1982. - S. 512.

An excerpt characterizing Uranium-235

In the 12th and 13th years, Kutuzov was directly accused of mistakes. The sovereign was dissatisfied with him. And in a story recently written by the highest command, it is said that Kutuzov was a cunning court liar who was afraid of the name of Napoleon and, with his mistakes near Krasnoye and near the Berezina, deprived the Russian troops of glory - a complete victory over the French. [History of 1812 by Bogdanovich: characterization of Kutuzov and discussion of the unsatisfactory results of the Krasnensky battles. (Note by L.N. Tolstoy.)]
Such is the fate not of great people, not grand homme, whom the Russian mind does not recognize, but the fate of those rare, always lonely people who, comprehending the will of Providence, subordinate their personal will to it. The hatred and contempt of the crowd punish these people for the enlightenment of higher laws.
For Russian historians - it is strange and terrible to say - Napoleon is the most insignificant tool of history - never and nowhere, even in exile, who did not show human dignity - Napoleon is an object of admiration and delight; he grand. Kutuzov, the man who, from the beginning to the end of his activity in 1812, from Borodin to Vilna, never betraying himself with a single action, not a word, is an extraordinary example of history of self-denial and awareness in the present of the future meaning of an event, - Kutuzov seems to them something indefinite and pathetic, and, speaking of Kutuzov and the 12th year, they always seem to be a little ashamed.
Meanwhile, it is difficult to imagine a historical person whose activity would be so invariably and constantly directed towards the same goal. It is difficult to imagine a goal more worthy and more in line with the will of the whole people. It is even more difficult to find another example in history where the goal set by a historical person would be so completely achieved as the goal towards which Kutuzov’s entire activity was directed in 1812.
Kutuzov never talked about the forty centuries that look from the pyramids, about the sacrifices that he brings to the fatherland, about what he intends to do or has done: he did not say anything at all about himself, did not play any role, he always seemed the most simple and ordinary man and said the most simple and ordinary things. He wrote letters to his daughters and m me Stael, read novels, loved the company of beautiful women, joked with generals, officers and soldiers, and never contradicted those people who wanted to prove something to him. When Count Rostopchin on the Yauzsky Bridge galloped up to Kutuzov with personal reproaches about who was to blame for the death of Moscow, and said: “How did you promise not to leave Moscow without giving a battle?” - Kutuzov answered: "I will not leave Moscow without a fight," despite the fact that Moscow had already been abandoned. When Arakcheev, who came to him from the sovereign, said that Yermolov should be appointed head of artillery, Kutuzov replied: “Yes, I just said it myself,” although he said something completely different in a minute. What did it matter to him, who alone then understood the whole enormous meaning of the event, among the stupid crowd that surrounded him, what did he care about whether Count Rostopchin would attribute the disaster of the capital to himself or to him? Even less could he be interested in who would be appointed chief of artillery.
Not only in these cases, but incessantly this old man, who by experience of life had reached the conviction that the thoughts and words that serve as their expression are not the essence of people's engines, spoke words that were completely meaningless - the first that came to his mind.
But this same man, who so neglected his words, never once in all his activity said a single word that would not be in accordance with the sole goal towards which he was going during the whole war. Obviously, involuntarily, with a heavy certainty that they would not understand him, he repeatedly expressed his opinion in the most diverse circumstances. Starting from the battle of Borodino, from which his discord with those around him began, he alone said that the battle of Borodino was a victory, and he repeated this verbally, and in reports, and reports until his death. He alone said that the loss of Moscow is not the loss of Russia. In response to Loriston's proposal for peace, he replied that there could be no peace, because such was the will of the people; he alone, during the retreat of the French, said that all our maneuvers were not needed, that everything would become better by itself than we wished, that the enemy should be given a golden bridge, that neither Tarutino, nor Vyazemsky, nor Krasnensky battles were needed, what with what someday you need to come to the border, that for ten Frenchmen he will not give up one Russian.
And he is alone, this court man, as he is portrayed to us, a man who lies to Arakcheev in order to please the sovereign - he alone, this court man, in Vilna, thereby deserving the sovereign's disfavor, says that further war abroad is harmful and useless.
But words alone would not prove that he then understood the significance of the event. His actions - all without the slightest retreat, all were directed towards the same goal, expressed in three actions: 1) strain all their forces to clash with the French, 2) defeat them and 3) expel them from Russia, facilitating, as far as possible, disasters of the people and troops.
He, that procrastinator Kutuzov, whose motto is patience and time, the enemy of decisive action, he gives the battle of Borodino, dressing the preparations for it in unparalleled solemnity. He, that Kutuzov, who in the battle of Austerlitz, before it began, says that it will be lost, in Borodino, despite the assurances of the generals that the battle is lost, despite the unheard-of example in history that after the battle won, the army must retreat , he alone, in opposition to everyone, claims until his death that the battle of Borodino is a victory. He alone during the entire retreat insists on not giving battles, which are now useless, not starting a new war and not crossing the borders of Russia.
Now it is easy to understand the meaning of an event, unless we apply to the activity of masses of goals that were in the head of a dozen people, since the whole event with its consequences lies before us.
But how then could this old man, alone, contrary to the opinion of all, guess, so correctly guessed then the meaning of the popular meaning of the event, that he never betrayed him in all his activity?
The source of this extraordinary power of insight into the meaning of occurring phenomena lay in that popular feeling, which he carried within himself in all its purity and strength.
Only the recognition of this feeling in him made the people, in such strange ways, from an old man who was in disfavor, choose him against the will of the tsar to be representatives of the people's war. And only this feeling put him on that highest human height, from which he, the commander-in-chief, directed all his forces not to kill and exterminate people, but to save and pity them.

Where did uranium come from? Most likely, it appears during supernova explosions. The fact is that for the nucleosynthesis of elements heavier than iron, there must be a powerful neutron flux, which occurs just during a supernova explosion. It would seem that later, when condensing from the cloud of new star systems formed by it, uranium, having gathered in a protoplanetary cloud and being very heavy, should sink into the depths of the planets. But it's not. Uranium is a radioactive element and it releases heat when it decays. The calculation shows that if uranium were evenly distributed throughout the entire thickness of the planet, at least with the same concentration as on the surface, then it would release too much heat. Moreover, its flow should decrease as uranium is consumed. Since nothing of the kind is observed, geologists believe that at least a third of uranium, and perhaps all of it, is concentrated in the earth's crust, where its content is 2.5∙10 -4%. Why this happened is not discussed.

Where is uranium mined? Uranium on Earth is not so small - in terms of prevalence, it is in 38th place. And most of all this element is in sedimentary rocks - carbonaceous shales and phosphorites: up to 8∙10 -3 and 2.5∙10 -2%, respectively. In total, the earth's crust contains 10 14 tons of uranium, but the main problem is that it is very dispersed and does not form powerful deposits. About 15 uranium minerals are of industrial importance. This is uranium pitch - its base is tetravalent uranium oxide, uranium mica - various silicates, phosphates and more complex compounds with vanadium or titanium based on hexavalent uranium.

What are Becquerel rays? After the discovery of X-rays by Wolfgang Roentgen, the French physicist Antoine-Henri Becquerel became interested in the glow of uranium salts, which occurs under the action of sunlight. He wanted to understand if there were X-rays here too. Indeed, they were present - the salt illuminated the photographic plate through the black paper. In one of the experiments, however, the salt was not illuminated, and the photographic plate still darkened. When a metal object was placed between the salt and the photographic plate, the darkening under it was less. Consequently, the new rays did not arise at all due to the excitation of uranium by light and did not partially pass through the metal. They were called at first "Becquerel rays". Subsequently, it was found that these are mainly alpha rays with a small addition of beta rays: the fact is that the main isotopes of uranium emit an alpha particle during decay, and the daughter products also experience beta decay.

How high is the radioactivity of uranium? Uranium has no stable isotopes, they are all radioactive. The longest-lived is uranium-238 with a half-life of 4.4 billion years. The next is uranium-235 - 0.7 billion years. Both of them undergo alpha decay and become the corresponding isotopes of thorium. Uranium-238 makes up over 99% of all natural uranium. Because of its long half-life, the radioactivity of this element is small, and besides, alpha particles are not able to overcome the stratum corneum on the surface of the human body. They say that IV Kurchatov, after working with uranium, simply wiped his hands with a handkerchief and did not suffer from any diseases associated with radioactivity.

Researchers have repeatedly turned to the statistics of diseases of workers in uranium mines and processing plants. For example, here is a recent article by Canadian and American experts who analyzed the health data of more than 17,000 workers at the Eldorado mine in the Canadian province of Saskatchewan for the years 1950-1999 ( environmental research, 2014, 130, 43–50, DOI:10.1016/j.envres.2014.01.002). They proceeded from the fact that radiation has the strongest effect on rapidly multiplying blood cells, leading to the corresponding types of cancer. Statistics also showed that mine workers have a lower incidence of various types of blood cancer than the average Canadian. At the same time, the main source of radiation is considered not uranium itself, but the gaseous radon generated by it and its decay products, which can enter the body through the lungs.

Why is uranium harmful?? It, like other heavy metals, is highly toxic and can cause kidney and liver failure. On the other hand, uranium, being a dispersed element, is inevitably present in water, soil and, concentrating in the food chain, enters the human body. It is reasonable to assume that in the process of evolution, living beings have learned to neutralize uranium in natural concentrations. The most dangerous uranium is in water, so the WHO set a limit: at first it was 15 µg/l, but in 2011 the standard was increased to 30 µg/g. As a rule, there is much less uranium in water: in the USA, on average, 6.7 μg / l, in China and France - 2.2 μg / l. But there are also strong deviations. So in some areas of California it is a hundred times more than the standard - 2.5 mg / l, and in Southern Finland it reaches 7.8 mg / l. Researchers are trying to understand whether the WHO standard is too strict by studying the effect of uranium on animals. Here is a typical job BioMed Research International, 2014, ID 181989; DOI:10.1155/2014/181989). French scientists fed rats for nine months with water supplemented with depleted uranium, and in a relatively high concentration - from 0.2 to 120 mg / l. The lower value is water near the mine, while the upper one is not found anywhere - the maximum concentration of uranium, measured in the same Finland, is 20 mg / l. To the surprise of the authors - the article is titled: "The unexpected absence of a noticeable effect of uranium on physiological systems ..." - uranium had practically no effect on the health of rats. The animals ate well, put on weight properly, did not complain of illness and did not die of cancer. Uranium, as it should be, was deposited primarily in the kidneys and bones, and in a hundredfold smaller amount - in the liver, and its accumulation, as expected, depended on the content in the water. However, this did not lead to kidney failure, or even to the noticeable appearance of any molecular markers of inflammation. The authors suggested starting a review of the stringent WHO guidelines. However, there is one caveat: the effect on the brain. There was less uranium in the brains of rats than in the liver, but its content did not depend on the amount in water. But uranium affected the work of the antioxidant system of the brain: the activity of catalase increased by 20%, glutathione peroxidase increased by 68–90%, while the activity of superoxide dismutase fell by 50% regardless of the dose. This means that uranium clearly caused oxidative stress in the brain and the body reacted to it. Such an effect - a strong effect of uranium on the brain in the absence of its accumulation in it, by the way, as well as in the genital organs - was noticed earlier. Moreover, water with uranium at a concentration of 75–150 mg/l, which researchers from the University of Nebraska fed to rats for six months ( Neurotoxicology and Teratology, 2005, 27, 1, 135–144; DOI:10.1016/j.ntt.2004.09.001) affected the behavior of animals, mainly males, released into the field: they crossed the lines, stood up on their hind legs, and brushed their fur, unlike the control ones. There is evidence that uranium also leads to memory impairment in animals. The change in behavior correlated with the level of lipid oxidation in the brain. It turns out that rats from uranium water became healthy, but stupid. These data will still be useful to us in the analysis of the so-called Persian Gulf syndrome (Gulf War Syndrome).

Does uranium pollute shale gas mining sites? It depends on how much uranium is in the gas-containing rocks and how it is associated with them. For example, Associate Professor Tracy Bank of the University at Buffalo has explored the Marcelus Shale, which stretches from western New York State through Pennsylvania and Ohio to West Virginia. It turned out that uranium is chemically bound precisely with the source of hydrocarbons (recall that related carbonaceous shales have the highest uranium content). Experiments have shown that the solution used for fracturing the seam perfectly dissolves uranium. “When the uranium in these waters is on the surface, it can cause pollution of the surrounding area. It does not carry a radiation risk, but uranium is a poisonous element, ”Tracey Bank notes in a university press release dated October 25, 2010. Detailed articles on the risk of environmental pollution with uranium or thorium during the extraction of shale gas have not yet been prepared.

Why is uranium needed? Previously, it was used as a pigment for the manufacture of ceramics and colored glass. Now uranium is the basis of nuclear energy and nuclear weapons. In this case, its unique property is used - the ability of the nucleus to divide.

What is nuclear fission? The disintegration of the nucleus into two unequal large pieces. It is precisely because of this property that during nucleosynthesis due to neutron irradiation, nuclei heavier than uranium are formed with great difficulty. The essence of the phenomenon is as follows. If the ratio of the number of neutrons and protons in the nucleus is not optimal, it becomes unstable. Usually such a nucleus ejects either an alpha particle - two protons and two neutrons, or a beta particle - a positron, which is accompanied by the transformation of one of the neutrons into a proton. In the first case, an element of the periodic table is obtained, spaced two cells back, in the second - one cell forward. However, the uranium nucleus, in addition to emitting alpha and beta particles, is capable of fission - decaying into the nuclei of two elements in the middle of the periodic table, such as barium and krypton, which it does, having received a new neutron. This phenomenon was discovered shortly after the discovery of radioactivity, when physicists exposed everything they had to the newly discovered radiation. Here is how Otto Frisch, a participant in the events, writes about this (Uspekhi fizicheskikh nauk, 1968, 96, 4). After the discovery of beryllium rays - neutrons - Enrico Fermi irradiated them, in particular, uranium to cause beta decay - he hoped to get the next, 93rd element, now called neptunium, at his expense. It was he who discovered a new type of radioactivity in irradiated uranium, which he associated with the appearance of transuranium elements. In this case, slowing down neutrons, for which the beryllium source was covered with a layer of paraffin, increased this induced radioactivity. The American radiochemist Aristide von Grosse suggested that one of these elements was protactinium, but he was wrong. But Otto Hahn, who was then working at the University of Vienna and considered protactinium discovered in 1917 to be his brainchild, decided that he was obliged to find out what elements were obtained in this case. Together with Lise Meitner, in early 1938, Hahn suggested, based on the results of experiments, that whole chains of radioactive elements are formed, arising from multiple beta decays of the nuclei of uranium-238 that absorbed the neutron and its daughter elements. Soon Lise Meitner was forced to flee to Sweden, fearing possible reprisals from the Nazis after the Anschluss of Austria. Gan, continuing his experiments with Fritz Strassmann, discovered that among the products there was also barium, element number 56, which could not have been obtained from uranium in any way: all chains of uranium alpha decays end in much heavier lead. The researchers were so surprised by the result that they did not publish it, they only wrote letters to friends, in particular Lise Meitner in Gothenburg. There, at Christmas 1938, her nephew, Otto Frisch, visited her, and, walking in the vicinity of the winter city - he is on skis, his aunt is on foot - they discussed the possibility of the appearance of barium during the irradiation of uranium due to nuclear fission (for more on Lise Meitner, see "Chemistry and Life ", 2013, No. 4). Returning to Copenhagen, Frisch, literally on the gangway of a steamer departing for the USA, caught Niels Bohr and informed him about the idea of ​​division. Bor, slapping his forehead, said: “Oh, what fools we were! We should have noticed this sooner." In January 1939, Frisch and Meitner published an article on the fission of uranium nuclei under the action of neutrons. By that time, Otto Frisch had already set up a control experiment, as well as many American groups that received a message from Bohr. They say that physicists began to disperse to their laboratories right during his report on January 26, 1939 in Washington at the annual conference on theoretical physics, when they grasped the essence of the idea. After the discovery of fission, Hahn and Strassmann revised their experiments and found, just like their colleagues, that the radioactivity of irradiated uranium is not associated with transuraniums, but with the decay of radioactive elements formed during fission from the middle of the periodic table.

How does a chain reaction work in uranium? Shortly after the possibility of fission of uranium and thorium nuclei was experimentally proved (and there are no other fissile elements on Earth in any significant amount), Niels Bohr and John Wheeler, who worked at Princeton, as well as independently the Soviet theoretical physicist Ya. I. Frenkel and the Germans Siegfried Flügge and Gottfried von Droste created the theory of nuclear fission. Two mechanisms followed from it. One is related to the threshold absorption of fast neutrons. According to him, to initiate fission, the neutron must have a rather high energy, more than 1 MeV for the nuclei of the main isotopes - uranium-238 and thorium-232. At lower energies, the absorption of a neutron by uranium-238 has a resonant character. Thus, a neutron with an energy of 25 eV has a capture cross section that is thousands of times larger than with other energies. In this case, there will be no fission: uranium-238 will become uranium-239, which with a half-life of 23.54 minutes will turn into neptunium-239, the one with a half-life of 2.33 days will turn into long-lived plutonium-239. Thorium-232 will become uranium-233.

The second mechanism is the non-threshold absorption of a neutron, followed by the third more or less common fissile isotope - uranium-235 (as well as plutonium-239 and uranium-233, which are absent in nature): by absorbing any neutron, even a slow one, the so-called thermal, with an energy of for molecules participating in thermal motion - 0.025 eV, such a nucleus will be divided. And this is very good: for thermal neutrons, the capture cross-sectional area is four times higher than for fast, megaelectronvolt ones. This is the significance of uranium-235 for the entire subsequent history of nuclear energy: it is it that ensures the multiplication of neutrons in natural uranium. After hitting a neutron, the uranium-235 nucleus becomes unstable and quickly splits into two unequal parts. Along the way, several (on average 2.75) new neutrons fly out. If they hit the nuclei of the same uranium, they will cause the neutrons to multiply exponentially - a chain reaction will start, which will lead to an explosion due to the rapid release of a huge amount of heat. Neither uranium-238 nor thorium-232 can work like this: after all, during fission, neutrons with an average energy of 1-3 MeV are emitted, that is, if there is an energy threshold of 1 MeV, a significant part of the neutrons will certainly not be able to cause a reaction, and there will be no reproduction. This means that these isotopes should be forgotten and neutrons will have to be slowed down to thermal energy so that they interact with uranium-235 nuclei as efficiently as possible. At the same time, their resonant absorption by uranium-238 cannot be allowed: after all, in natural uranium this isotope is slightly less than 99.3%, and neutrons more often collide with it, and not with the target uranium-235. And acting as a moderator, it is possible to maintain neutron multiplication at a constant level and prevent an explosion - to control a chain reaction.

The calculation carried out by Ya. B. Zeldovich and Yu. B. Khariton in the same fateful 1939 showed that for this it is necessary to use a neutron moderator in the form of heavy water or graphite and enrich natural uranium with uranium-235 by at least 1.83 times. Then this idea seemed to them pure fantasy: “It should be noted that approximately double the enrichment of those fairly significant amounts of uranium that are necessary to carry out a chain explosion,<...>is an extremely cumbersome task, close to practical impossibility." Now this problem has been solved, and the nuclear industry is mass-producing uranium enriched with uranium-235 up to 3.5% for power plants.

What is spontaneous nuclear fission? In 1940, G. N. Flerov and K. A. Petrzhak discovered that uranium fission can occur spontaneously, without any external influence, although the half-life is much longer than with ordinary alpha decay. Since such fission also produces neutrons, if they are not allowed to fly away from the reaction zone, they will serve as the initiators of the chain reaction. It is this phenomenon that is used in the creation of nuclear reactors.

Why is nuclear power needed? Zel'dovich and Khariton were among the first to calculate the economic effect of nuclear energy (Uspekhi fizicheskikh nauk, 1940, 23, 4). “... At the moment, it is still impossible to make final conclusions about the possibility or impossibility of implementing a nuclear fission reaction in uranium with infinitely branching chains. If such a reaction is feasible, then the reaction rate is automatically adjusted to ensure that it proceeds smoothly, despite the huge amount of energy at the disposal of the experimenter. This circumstance is exceptionally favorable for the energy utilization of the reaction. Therefore, although this is a division of the skin of an unkilled bear, we present some numbers that characterize the possibilities for the energy use of uranium. If the fission process proceeds on fast neutrons, therefore, the reaction captures the main isotope of uranium (U238), then<исходя из соотношения теплотворных способностей и цен на уголь и уран>the cost of a calorie from the main isotope of uranium turns out to be about 4000 times cheaper than from coal (unless, of course, the processes of "burning" and heat removal turn out to be much more expensive in the case of uranium than in the case of coal). In the case of slow neutrons, the cost of a "uranium" calorie (based on the above figures) will, taking into account that the abundance of the isotope U235 is 0.007, is already only 30 times cheaper than a "coal" calorie, all other things being equal.

The first controlled chain reaction was carried out in 1942 by Enrico Fermi at the University of Chicago, and the reactor was manually controlled by pushing and pulling out graphite rods as the neutron flux changed. The first power plant was built in Obninsk in 1954. In addition to generating energy, the first reactors also worked to produce weapons-grade plutonium.

How does a nuclear power plant work? Most reactors now operate on slow neutrons. Enriched uranium in the form of a metal, an alloy, for example with aluminum, or in the form of an oxide is put into long cylinders - fuel elements. They are installed in a certain way in the reactor, and rods from the moderator are introduced between them, which control the chain reaction. Over time, reactor poisons accumulate in the fuel element - uranium fission products, also capable of absorbing neutrons. When the uranium-235 concentration falls below the critical level, the element is decommissioned. However, it contains many fission fragments with strong radioactivity, which decreases over the years, which is why the elements emit a significant amount of heat for a long time. They are kept in cooling pools, and then they are either buried or they try to process them - to extract unburned uranium-235, accumulated plutonium (it was used to make atomic bombs) and other isotopes that can be used. The unused part is sent to the burial grounds.

In so-called fast neutron reactors, or breeder reactors, reflectors of uranium-238 or thorium-232 are installed around the elements. They slow down and send too fast neutrons back to the reaction zone. Slowed down to resonant speeds, neutrons absorb these isotopes, turning into plutonium-239 or uranium-233, respectively, which can serve as fuel for a nuclear power plant. Since fast neutrons do not react well with uranium-235, it is necessary to significantly increase its concentration, but this pays off with a stronger neutron flux. Despite the fact that breeder reactors are considered the future of nuclear energy, since they provide more nuclear fuel than they consume, experiments have shown that they are difficult to manage. Now there is only one such reactor left in the world - at the fourth power unit of the Beloyarsk NPP.

How is nuclear energy criticized? If we don't talk about accidents, the main point in the arguments of the opponents of nuclear energy today was the proposal to add to the calculation of its efficiency the costs of protecting the environment after decommissioning the plant and when working with fuel. In both cases, the task of reliable disposal of radioactive waste arises, and these are the costs that the state bears. There is an opinion that if they are shifted to the cost of energy, then its economic attractiveness will disappear.

There is also opposition among supporters of nuclear energy. Its representatives point to the uniqueness of uranium-235, which has no replacement, because alternative isotopes fissile with thermal neutrons - plutonium-239 and uranium-233 - are absent in nature due to a half-life of thousands of years. And they are obtained just as a result of the fission of uranium-235. If it ends, an excellent natural source of neutrons for a nuclear chain reaction will disappear. As a result of such extravagance, mankind will lose the opportunity in the future to involve thorium-232 in the energy cycle, the reserves of which are several times greater than those of uranium.

Theoretically, particle accelerators can be used to obtain a flux of fast neutrons with megaelectronvolt energies. However, if we are talking, for example, about interplanetary flights on an atomic engine, then it will be very difficult to implement a scheme with a bulky accelerator. The exhaustion of uranium-235 puts an end to such projects.

What is weapon-grade uranium? This is highly enriched uranium-235. Its critical mass - it corresponds to the size of a piece of matter in which a chain reaction spontaneously occurs - is small enough to make a munition. Such uranium can be used to make an atomic bomb, as well as a fuse for a thermonuclear bomb.

What disasters are associated with the use of uranium? The energy stored in the nuclei of fissile elements is enormous. Having escaped from control due to an oversight or due to intent, this energy can do a lot of trouble. The two worst nuclear disasters occurred on August 6 and 8, 1945, when the US Air Force dropped atomic bombs on Hiroshima and Nagasaki, killing and injuring hundreds of thousands of civilians. Catastrophes of a smaller scale are associated with accidents at nuclear power plants and nuclear cycle enterprises. The first major accident happened in 1949 in the USSR at the Mayak plant near Chelyabinsk, where plutonium was produced; liquid radioactive waste got into the river Techa. In September 1957, an explosion occurred on it with the release of a large amount of radioactive material. Eleven days later, the British plutonium reactor at Windscale burned down, a cloud of explosion products dissipated over Western Europe. In 1979, the reactor at the Trimail Island nuclear power plant in Pennsylvania burned down. The accidents at the Chernobyl nuclear power plant (1986) and the nuclear power plant in Fukushima (2011) led to the most widespread consequences, when millions of people were exposed to radiation. The first littered vast lands, throwing out 8 tons of uranium fuel with decay products as a result of the explosion, which spread throughout Europe. The second polluted and, three years after the accident, continues to pollute the Pacific Ocean in the areas of fisheries. The elimination of the consequences of these accidents was very expensive, and if these costs were decomposed into the cost of electricity, it would increase significantly.

A separate issue is the consequences for human health. According to official statistics, many people who survived the bombing or live in contaminated areas benefited from exposure - the former have a higher life expectancy, the latter have fewer cancers, and experts attribute a certain increase in mortality to social stress. The number of people who died precisely from the consequences of accidents or as a result of their liquidation is estimated at hundreds of people. Opponents of nuclear power plants point out that accidents have led to several million premature deaths on the European continent, they are simply invisible against the statistical background.

The withdrawal of lands from human use in accident zones leads to an interesting result: they become a kind of reserves, where biodiversity grows. True, some animals suffer from diseases associated with radiation. The question of how quickly they will adapt to the increased background remains open. There is also an opinion that the consequence of chronic irradiation is “selection for a fool” (see Chemistry and Life, 2010, No. 5): more primitive organisms survive even at the embryonic stage. In particular, in relation to people, this should lead to a decrease in the mental abilities of the generation born in the contaminated territories shortly after the accident.

What is depleted uranium? This is uranium-238 left over from the extraction of uranium-235. The volumes of waste from the production of weapons-grade uranium and fuel elements are large - in the United States alone, 600 thousand tons of such uranium hexafluoride have accumulated (for problems with it, see "Chemistry and Life", 2008, No. 5). The content of uranium-235 in it is 0.2%. These wastes must either be stored until better times, when fast neutron reactors will be created and it will be possible to process uranium-238 into plutonium, or somehow used.

They found a use for it. Uranium, like other transition elements, is used as a catalyst. For example, the authors of an article in ACS Nano dated June 30, 2014, they write that a uranium or thorium catalyst with graphene for the reduction of oxygen and hydrogen peroxide "has great potential for energy applications." Because of its high density, uranium serves as ballast for ships and counterweights for aircraft. This metal is also suitable for radiation protection in medical devices with radiation sources.

What weapons can be made from depleted uranium? Bullets and cores for armor-piercing projectiles. Here is the calculation. The heavier the projectile, the higher its kinetic energy. But the larger the projectile, the less concentrated its impact. This means that heavy metals with a high density are needed. Bullets are made of lead (Ural hunters at one time also used native platinum, until they realized that it was a precious metal), while the cores of the shells were made of a tungsten alloy. Conservationists point out that lead pollutes the soil in places of war or hunting and it would be better to replace it with something less harmful, for example, with the same tungsten. But tungsten is not cheap, and uranium, similar in density to it, is a harmful waste. At the same time, the permissible contamination of soil and water with uranium is approximately twice as high as for lead. This happens because the weak radioactivity of depleted uranium (and it is also 40% less than that of natural uranium) is neglected and a really dangerous chemical factor is taken into account: uranium, as we remember, is poisonous. At the same time, its density is 1.7 times greater than that of lead, which means that the size of uranium bullets can be reduced by half; uranium is much more refractory and harder than lead - when fired, it evaporates less, and when it hits a target, it produces fewer microparticles. In general, a uranium bullet pollutes the environment less than a lead one, however, this use of uranium is not known for certain.

But it is known that depleted uranium plates are used to strengthen the armor of American tanks (this is facilitated by its high density and melting point), and also instead of tungsten alloy in cores for armor-piercing projectiles. The uranium core is also good because uranium is pyrophoric: its hot small particles, formed when they hit the armor, flare up and set fire to everything around. Both applications are considered radiation safe. So, the calculation showed that, even after spending a year without getting out in a tank with uranium armor loaded with uranium ammunition, the crew would receive only a quarter of the allowable dose. And in order to obtain an annual allowable dose, such ammunition must be screwed to the surface of the skin for 250 hours.

Projectiles with uranium cores - for 30-mm aircraft guns or for artillery sub-calibers - were used by the Americans in recent wars, starting with the 1991 Iraq campaign of the year. That year, they poured 300 tons of depleted uranium on Iraqi armored units in Kuwait, and during their retreat, 250 tons, or 780,000 rounds, fell on aircraft guns. In Bosnia and Herzegovina, during the bombing of the army of the unrecognized Republika Srpska, 2.75 tons of uranium were used, and during the shelling of the Yugoslav army in the province of Kosovo and Metohija - 8.5 tons, or 31,000 rounds. Since the WHO had by that time taken care of the consequences of the use of uranium, monitoring was carried out. He showed that one volley consisted of approximately 300 rounds, of which 80% contained depleted uranium. 10% hit the targets, and 82% fell within 100 meters of them. The rest dispersed within 1.85 km. The shell that hit the tank burned down and turned into an aerosol, light targets like armored personnel carriers were pierced through by a uranium shell. Thus, one and a half tons of shells could turn into uranium dust in Iraq at the most. According to experts from the American strategic research center RAND Corporation, more than 10 to 35% of the used uranium has turned into an aerosol. Croatian uranium munitions fighter Asaf Durakovich, who has worked in a variety of organizations from the King Faisal Hospital in Riyadh to the Washington Uranium Medical Research Center, believes that in southern Iraq alone in 1991, 3-6 tons of submicron uranium particles were formed, which scattered over a wide area , that is, uranium pollution there is comparable to Chernobyl.

Unlike the other, most common uranium isotope 238U, a self-sustaining nuclear chain reaction is possible in 235U. Therefore, this isotope is used as fuel in nuclear reactors, as well as in nuclear weapons.

The activity of one gram of this nuclide is approximately 80 kBq.

• 1 Formation and breakup
• 2 Forced division
  • 2.1 Nuclear chain reaction
• 3 Isomers
• 4 Application
• 5 See also
• 6 Notes

Formation and decay

Uranium-235 is formed as a result of the following decays:

• β-decay of the nuclide 235Pa (half-life is 24.44(11) min):

• K-capture by nuclide 235Np (half-life is 396.1(12) days):

• α-decay of the nuclide 239Pu (half-life is 2.411(3) 104 years):

The decay of uranium-235 occurs in the following ways:

• α-decay in 231Th (probability 100%, decay energy 4678.3(7) keV):

• Spontaneous fission (probability 7(2) 10−9%);
• Cluster decay with the formation of nuclides 20Ne, 25Ne and 28Mg (the probabilities are respectively 8(4) 10−10%, 8 10−10%, 8 10−10%):

Forced division

In the early 1930s Enrico Fermi carried out the irradiation of uranium with neutrons, with the aim of obtaining transuranium elements in this way. But in 1939, O. Hahn and F. Strassmann were able to show that when a neutron is absorbed by a uranium nucleus, a forced fission reaction occurs. As a rule, the nucleus is divided into two fragments, with the release of 2-3 neutrons (see diagram).

About 300 isotopes of various elements were found in the fission products of uranium-235: from Z=30 (zinc) to Z=64 (gadolinium). The dependence curve of the relative yield of isotopes formed during irradiation of uranium-235 with slow neutrons on the mass number is symmetrical and resembles the letter "M" in shape. The two pronounced maxima of this curve correspond to mass numbers 95 and 134, and the minimum falls on the range of mass numbers from 110 to 125. Thus, the fission of uranium into fragments of equal mass (with mass numbers 115-119) occurs with a lower probability than asymmetric fission, such a tendency is observed in all fissile isotopes and is not associated with any individual properties of nuclei or particles, but is inherent in the very mechanism of nuclear fission. However, the asymmetry decreases as the excitation energy of the fissile nucleus increases, and at a neutron energy of more than 100 MeV, the mass distribution of fission fragments has one maximum corresponding to symmetric fission of the nucleus.

One of the options for forced fission of uranium-235 after the absorption of a neutron (scheme)

The fragments formed during the fission of the uranium nucleus, in turn, are radioactive, and undergo a chain of β-decays, in which additional energy is gradually released over a long time. The average energy released during the decay of one uranium-235 nucleus, taking into account the decay of fragments, is approximately 202.5 MeV = 3.244 10−11 J, or 19.54 TJ/mol = 83.14 TJ/kg.

Nuclear fission is just one of the many processes that are possible during the interaction of neutrons with nuclei; it is he who underlies the operation of any nuclear reactor.

Nuclear chain reaction

The decay of one 235U nucleus usually emits from 1 to 8 (2.5 on average) free neutrons. Each neutron formed during the decay of the 235U nucleus, subject to interaction with another 235U nucleus, can cause a new decay event, this phenomenon is called a nuclear fission chain reaction.

Hypothetically, the number of neutrons of the second generation (after the second stage of nuclear decay) can exceed 3² = 9. With each subsequent stage of the fission reaction, the number of neutrons produced can grow like an avalanche. In real conditions, free neutrons may not generate a new fission event, leaving the sample before the capture of 235U, or being captured both by the 235U isotope itself with its transformation into 236U, and by other materials (for example, 238U, or by the resulting nuclear fission fragments, such as 149Sm or 135Xe ).

If, on average, each fission generates another new fission, then the reaction becomes self-sustaining; this condition is called critical. (see also neutron multiplication factor)

In real conditions, reaching the critical state of uranium is not so easy, since a number of factors affect the course of the reaction. For example, natural uranium consists of only 0.72% 235U, 99.2745% is 238U, which absorbs neutrons produced during the fission of 235U nuclei. This leads to the fact that in natural uranium at present the fission chain reaction decays very quickly. An undamped fission chain reaction can be carried out in several main ways:

• Increase the volume of the sample (for uranium extracted from the ore, it is possible to achieve a critical mass due to an increase in volume);
• Carry out isotope separation by increasing the concentration of 235U in the sample;
• Reduce the loss of free neutrons through the surface of the sample by using various types of reflectors;
• Use a substance - neutron moderator to increase the concentration of thermal neutrons.

Isomers

A single 235Um isomer is known with the following characteristics:

• Mass excess: 40920.6(1.8) keV
• Excitation energy: 76.5(4) eV
• Half-life: 26 min
• Spin and parity of the nucleus: 1/2+

The decay of the isomeric state is carried out by isomeric transition to the ground state.

Application

• Uranium-235 is used as fuel for nuclear reactors in which a controlled fission chain reaction is carried out;
• Uranium with a high degree of enrichment is used to create nuclear weapons. In this case, an uncontrolled nuclear chain reaction is used to release a large amount of energy (an explosion).

see also

• Isotopes of uranium
• Isotope separation

Notes

1. 1 2 3 4 5 G.Audi, A.H. Wapstra, and C. Thibault (2003). "The AME2003 atomic mass evaluation (II). Tables, graphs, and references. Nuclear Physics A 729 : 337-676. DOI:10.1016/j.nuclphysa.2003.11.003. Bibcode: 2003NuPhA.729..337A.
2. 1 2 3 4 5 6 7 8 9 10 11 12 G. Audi, O. Bersillon, J. Blachot and A. H. Wapstra (2003). "The NUBASE evaluation of nuclear and decay properties". Nuclear Physics A 729 : 3–128. DOI:10.1016/j.nuclphysa.2003.11.001. Bibcode: 2003NuPhA.729....3A.
3. Hoffman K. Is it possible to make gold? - 2nd ed. erased - L.: Chemistry, 1987. - S. 130. - 232 p. - 50,000 copies.
4. Today in science history
5. 1 2 3 Fialkov Yu. Ya. Application of isotopes in chemistry and chemical industry. - Kyiv: Tehnika, 1975. - S. 87. - 240 p. - 2,000 copies.
6. Table of Physical and Chemical Constants, Sec 4.7.1: Nuclear Fission. Kaye & Laby Online. Archived from the original on April 8, 2012.
7. Bartolomey GG, Baibakov VD, Alkhutov MS, Bat' GA Fundamentals of theory and calculation methods for nuclear power reactors. - M.: Energoatomizdat, 1982. - S. 512.

uranium 235 50, uranium 235 75, uranium 235 area, uranium 235/75r15

Uranium is a chemical element of the actinide family with atomic number 92. It is the most important nuclear fuel. Its concentration in the earth's crust is about 2 parts per million. Important uranium minerals include uranium oxide (U 3 O 8), uraninite (UO 2), carnotite (potassium uranyl vanadate), otenite (potassium uranyl phosphate), and torbernite (hydrous copper and uranyl phosphate). These and other uranium ores are sources of nuclear fuel and contain many times more energy than all known recoverable fossil fuel deposits. 1 kg of uranium 92 U gives as much energy as 3 million kg of coal.

Discovery history

The chemical element uranium is a dense, solid silver-white metal. It is ductile, malleable and can be polished. Metal oxidizes in air and ignites when crushed. Relatively poor conductor of electricity. The electronic formula of uranium is 7s2 6d1 5f3.

Although the element was discovered in 1789 by the German chemist Martin Heinrich Klaproth, who named it after the newly discovered planet Uranus, the metal itself was isolated in 1841 by the French chemist Eugène-Melchior Peligot by reduction from uranium tetrachloride (UCl 4 ) with potassium.

Radioactivity

The creation of the periodic table by the Russian chemist Dmitri Mendeleev in 1869 focused attention on uranium as the heaviest known element, which it remained until the discovery of neptunium in 1940. In 1896, the French physicist Henri Becquerel discovered the phenomenon of radioactivity in it. This property was later found in many other substances. It is now known that radioactive uranium in all its isotopes consists of a mixture of 238 U (99.27%, half-life - 4,510,000,000 years), 235 U (0.72%, half-life - 713,000,000 years) and 234 U (0.006%, half-life - 247,000 years). This makes it possible, for example, to determine the age of rocks and minerals in order to study geological processes and the age of the Earth. To do this, they measure the amount of lead, which is the end product of the radioactive decay of uranium. In this case, 238 U is the initial element, and 234 U is one of the products. 235 U gives rise to actinium decay series.

Opening a chain reaction

The chemical element uranium became the subject of wide interest and intensive study after the German chemists Otto Hahn and Fritz Strassmann discovered nuclear fission in it at the end of 1938 when bombarding it with slow neutrons. In early 1939, the American physicist of Italian origin Enrico Fermi suggested that among the products of the fission of the atom there may be elementary particles capable of generating a chain reaction. In 1939, the American physicists Leo Szilard and Herbert Anderson, as well as the French chemist Frederic Joliot-Curie and their colleagues, confirmed this prediction. Subsequent studies have shown that, on average, 2.5 neutrons are released during the fission of an atom. These discoveries led to the first self-sustaining nuclear chain reaction (12/02/1942), the first atomic bomb (07/16/1945), its first use in military operations (08/06/1945), the first nuclear submarine (1955) and the first full-scale nuclear power plant ( 1957).

Oxidation states

The chemical element uranium, being a strong electropositive metal, reacts with water. It dissolves in acids, but not in alkalis. Important oxidation states are +4 (as in UO 2 oxide, tetrahalides such as UCl 4 , and the green water ion U 4+) and +6 (as in UO 3 oxide, UF 6 hexafluoride, and UO 2 2+ uranyl ion). In an aqueous solution, uranium is most stable in the composition of the uranyl ion, which has a linear structure [O = U = O] 2+ . The element also has +3 and +5 states, but they are unstable. Red U 3+ oxidizes slowly in water that does not contain oxygen. The color of the UO 2 + ion is unknown because it undergoes disproportionation (UO 2 + is simultaneously reduced to U 4+ and oxidized to UO 2 2+ ) even in very dilute solutions.

Nuclear fuel

When exposed to slow neutrons, the fission of the uranium atom occurs in the relatively rare isotope 235 U. This is the only natural fissile material, and it must be separated from the isotope 238 U. However, after absorption and negative beta decay, uranium-238 turns into a synthetic element plutonium, which is split by the action of slow neutrons. Therefore, natural uranium can be used in converter and breeder reactors, in which fission is supported by rare 235 U and plutonium is produced simultaneously with the transmutation of 238 U. Fissile 233 U can be synthesized from the thorium-232 isotope, which is widespread in nature, for use as nuclear fuel. Uranium is also important as the primary material from which synthetic transuranium elements are obtained.

Other uses of uranium

Compounds of the chemical element were previously used as dyes for ceramics. Hexafluoride (UF 6) is a solid with an unusually high vapor pressure (0.15 atm = 15,300 Pa) at 25 °C. UF 6 is chemically very reactive, but despite its corrosive nature in the vapor state, UF 6 is widely used in gas diffusion and gas centrifuge methods to obtain enriched uranium.

Organometallic compounds are an interesting and important group of compounds in which metal-carbon bonds connect a metal to organic groups. Uranocene is an organouranium compound U(C 8 H 8) 2 in which the uranium atom is sandwiched between two layers of organic rings bonded to C 8 H 8 cyclooctatetraene. Its discovery in 1968 opened up a new field of organometallic chemistry.

Depleted natural uranium is used as a means of radiation protection, ballast, in armor-piercing projectiles and tank armor.

Recycling

The chemical element, although very dense (19.1 g / cm 3), is a relatively weak, non-flammable substance. Indeed, the metallic properties of uranium seem to place it somewhere between silver and other true metals and non-metals, so it is not used as a structural material. The main value of uranium lies in the radioactive properties of its isotopes and their ability to fission. In nature, almost all (99.27%) of the metal consists of 238 U. The rest is 235 U (0.72%) and 234 U (0.006%). Of these natural isotopes, only 235 U is directly fissioned by neutron irradiation. However, when 238 U is absorbed, it forms 239 U, which eventually decays into 239 Pu, a fissile material of great importance for nuclear energy and nuclear weapons. Another fissile isotope, 233 U, can be produced by neutron irradiation with 232 Th.

crystalline forms

The characteristics of uranium cause it to react with oxygen and nitrogen even under normal conditions. At higher temperatures, it reacts with a wide range of alloying metals to form intermetallic compounds. The formation of solid solutions with other metals is rare due to the special crystal structures formed by the atoms of the element. Between room temperature and a melting point of 1132 °C, uranium metal exists in 3 crystalline forms known as alpha (α), beta (β) and gamma (γ). The transformation from α- to β-state occurs at 668 °C and from β to γ ​​- at 775 °C. γ-uranium has a body-centered cubic crystal structure, while β has a tetragonal one. The α phase consists of layers of atoms in a highly symmetrical orthorhombic structure. This anisotropic distorted structure prevents the alloying metal atoms from replacing the uranium atoms or occupying the space between them in the crystal lattice. It was found that only molybdenum and niobium form solid solutions.

Ores

The Earth's crust contains about 2 parts per million of uranium, which indicates its wide distribution in nature. The oceans are estimated to contain 4.5 x 109 tons of this chemical element. Uranium is an important constituent of over 150 different minerals and a minor constituent of another 50. Primary minerals found in igneous hydrothermal veins and in pegmatites include uraninite and its variety pitchblende. In these ores, the element occurs in the form of dioxide, which, due to oxidation, can vary from UO 2 to UO 2.67. Other economically significant products from uranium mines are autunite (hydrated calcium uranyl phosphate), tobernite (hydrated copper uranyl phosphate), coffinite (black hydrated uranium silicate), and carnotite (hydrated potassium uranyl vanadate).

It is estimated that more than 90% of known low-cost uranium reserves are found in Australia, Kazakhstan, Canada, Russia, South Africa, Niger, Namibia, Brazil, China, Mongolia and Uzbekistan. Large deposits are found in the conglomerate rock formations of Elliot Lake, located north of Lake Huron in Ontario, Canada, and in the South African Witwatersrand gold mine. Sand formations in the Colorado Plateau and in the Wyoming Basin of the western United States also contain significant uranium reserves.

Mining

Uranium ores are found both in near-surface and deep (300-1200 m) deposits. Underground, the seam thickness reaches 30 m. As in the case of ores of other metals, uranium mining at the surface is carried out by large earth-moving equipment, and the development of deep deposits is carried out by traditional methods of vertical and inclined mines. The world production of uranium concentrate in 2013 amounted to 70 thousand tons. The most productive uranium mines are located in Kazakhstan (32% of the total production), Canada, Australia, Niger, Namibia, Uzbekistan and Russia.

Uranium ores usually contain only a small amount of uranium-bearing minerals, and they cannot be smelted by direct pyrometallurgical methods. Instead, hydrometallurgical procedures should be used to extract and purify uranium. Increasing the concentration greatly reduces the load on the processing circuits, but none of the conventional beneficiation methods commonly used for mineral processing, such as gravity, flotation, electrostatic and even hand sorting, are applicable. With few exceptions, these methods result in a significant loss of uranium.

Burning

The hydrometallurgical processing of uranium ores is often preceded by a high-temperature calcination step. Firing dehydrates the clay, removes carbonaceous materials, oxidizes sulfur compounds to harmless sulfates, and oxidizes any other reducing agents that may interfere with subsequent processing.

Leaching

Uranium is extracted from roasted ores with both acidic and alkaline aqueous solutions. For all leaching systems to function successfully, the chemical element must either initially be present in the more stable 6-valent form or be oxidized to this state during processing.

Acid leaching is usually carried out by stirring the mixture of ore and lixiviant for 4-48 hours at ambient temperature. Except in special circumstances, sulfuric acid is used. It is served in quantities sufficient to obtain the final liquor at pH 1.5. Sulfuric acid leaching schemes typically use either manganese dioxide or chlorate to oxidize tetravalent U 4+ to 6-valent uranyl (UO 2 2+). As a rule, about 5 kg of manganese dioxide or 1.5 kg of sodium chlorate per ton is sufficient for the oxidation of U 4+. In any case, oxidized uranium reacts with sulfuric acid to form the 4- uranyl sulfate complex anion.

Ore containing a significant amount of basic minerals such as calcite or dolomite is leached with a 0.5-1 molar sodium carbonate solution. Although various reagents have been studied and tested, the main oxidizing agent for uranium is oxygen. Ores are usually leached in air at atmospheric pressure and at a temperature of 75-80 °C for a period of time that depends on the specific chemical composition. Alkali reacts with uranium to form a readily soluble complex ion 4-.

Before further processing, solutions resulting from acid or carbonate leaching must be clarified. Large-scale separation of clays and other ore slurries is accomplished through the use of effective flocculating agents, including polyacrylamides, guar gum, and animal glue.

Extraction

Complex ions 4- and 4- can be sorbed from their respective leaching solutions of ion exchange resins. These special resins, characterized by their sorption and elution kinetics, particle size, stability and hydraulic properties, can be used in various processing technologies, such as fixed and moving bed, basket type and continuous slurry ion exchange resin method. Usually, solutions of sodium chloride and ammonia or nitrates are used to elute adsorbed uranium.

Uranium can be isolated from acid ore liquors by solvent extraction. In industry, alkyl phosphoric acids, as well as secondary and tertiary alkylamines, are used. As a general rule, solvent extraction is preferred over ion exchange methods for acidic filtrates containing more than 1 g/l uranium. However, this method is not applicable to carbonate leaching.

The uranium is then purified by dissolving in nitric acid to form uranyl nitrate, extracted, crystallized and calcined to form UO 3 trioxide. The reduced UO2 dioxide reacts with hydrogen fluoride to form tetrafluoride UF4, from which metallic uranium is reduced by magnesium or calcium at a temperature of 1300 °C.

Tetrafluoride can be fluorinated at 350 °C to form UF 6 hexafluoride, which is used to separate enriched uranium-235 by gas diffusion, gas centrifugation, or liquid thermal diffusion.

Uranus. Natural uranium consists of a mixture of three isotopes: uranium-234, uranium-235, uranium-238. Artificial radioactive - with mass numbers 227-240. The half-life of uranium-235 is 7x108 years, uranium-238 is 4.5x109 years. During the decay of uranium and daughter radionuclides, alpha and beta radiation, as well as gamma quanta, are emitted. Uranium penetrates the body in different ways, including through the skin. Soluble compounds are rapidly absorbed into the bloodstream and distributed throughout organs and tissues, accumulating in the kidneys, bones, liver, and spleen. The biological half-life from the lungs is 118-150 days, from the skeleton - 450 days. Due to uranium and its decay products, the annual is 1.34 mSv.

Thorium. Thorium-232 is an inert gas. Its decay products are solid radioactive substances. The half-life is 1.4x1010 years. During the transformations of thorium and its decay products, alpha-beta particles, as well as gamma quanta, are released. The mineral thorianite contains up to 45-88% thorium. Fuel rods are made from an alloy of thorium with enriched uranium. It enters the body through the lungs, gastrointestinal tract, and skin. Accumulates in the bone marrow, spleen. The biological half-life from most organs is 700 days, from the skeleton - 68 years.

Radium. Radium-226 is the most important radioactive decay product of uranium-238. Half-life 1622. It is a silvery white metal. Widely used in medicine as a source of alpha particles for radiation therapy. Enters the body through the respiratory system, gastrointestinal tract and skin. Most of the incoming radium is deposited in the skeleton. The biological half-life from the bones is about 17 years, from the lungs - 180 days, from other organs it is excreted in the first two days. When it enters the human body, it causes damage to bone tissue, red bone marrow, which leads to impaired hematopoiesis, fractures, and the development of tumors. During one day, 1 g of radium yields 1 mm3 of radon during decay.

Radon. Radon-222 is a colorless, odorless gas. The half-life is 3.83 days. Decay product of radium-226. Radon is an alpha emitter. It is formed in uranium deposits in radioactive ores, is contained in natural gas, groundwater, etc. It can also come out through cracks in rocks, in poorly ventilated mines, mines, its concentration can reach large values. Radon is found in many building materials. It also enters the atmosphere during volcanic activity, in the production of phosphates, and in the operation of geothermal power stations.

For medicinal purposes, it is used in the form of radon baths in the treatment of diseases of the joints, bones, peripheral nervous system, chronic gynecological diseases, etc. It is also used in the form of inhalations, irrigation, ingestion of water containing radon. It enters the body mainly through the respiratory system. The half-life of the body within a day. Radon gives ¾ of the annual equivalent dose from terrestrial radiation sources, and about ½ of the dose from all natural radiation sources.

Potassium. Potassium-40 is a silvery-white metal, it does not occur in free form, as it is very chemically active. Half life
1.32 x 109 years. When it decays, it emits a beta particle. It is a typical biological element. A person's need for potassium is 2-3 mg per kg of body weight per day. A lot of potassium is found in potatoes, beets, tomatoes. In the body, 100% of the incoming potassium is absorbed, distributed evenly throughout all organs, relatively more of it is in the liver and spleen. The half-life is about 60 days.

Iodine. Iodine-131 is formed in the fission reactions of uranium and plutonium, as well as when tellurium is irradiated with neutrons. The half-life is 8.05 days. It enters the body through the respiratory system, the gastrointestinal tract (100% of the incoming iodine is absorbed), and the skin. Accumulates mainly in the thyroid gland, its concentration in the gland is 200 times higher than in other tissues. Decaying, iodine releases a beta particle and 2 gamma rays. The elimination half-life from the thyroid gland is 138 days, from other organs 10-15 days. From the body of a pregnant woman, iodine passes through the placenta to the fetus.

Cesium. Cesium-137 makes a decisive contribution to the total equivalent dose of radiation. Cesium is a silvery white metal. It is a source of beta and gamma radiation. Half-life of cesium-137 -
30 years. Prior to the Chernobyl accident, nuclear explosions were the main source of cesium entering the environment. Most of the precipitated cesium is in a form that is easily absorbed. In plants, it mainly accumulates in straw and haulm. In the intestine, 100% of the incoming cesium is absorbed. It accumulates mainly in muscle tissue. The elimination half-life from the muscles is 140 days.

Strontium. Strontium-90 - half-life - 28.6 years (for strontium-89 - 50.5 days). Strontium-90 is a beta emitter. Strontium is easily absorbed by plants, animals, and humans. The strontium concentrator is corn, the content of strontium in it is 5-20 times higher than in the soil. In the human body, depending on the diet, from 5% to 100% of the incoming strontium is absorbed in the gastrointestinal tract (30% on average). Accumulates mainly in the skeleton. The maximum concentration is observed in children under 1 year old. The half-life of strontium from soft tissues is up to 10 days, from bones - up to 8-10 years.

Plutonium. Plutonium-239 is an alpha emitter. Its half-life is 24360 years. It is a silvery white metal. The source of plutonium is nuclear explosions, as well as nuclear power plant reactors, especially accidental releases. It is found in the soil in the surface layers and bottom sediments of water bodies. It enters the body through the lungs and gastrointestinal tract, and is absorbed from the gastrointestinal tract - much less than 1%. It accumulates in the lungs, liver, bone tissue. The elimination half-life from the skeleton is 100 years, from the liver - 40 years.

Americium. Americium-241 is a decay product of plutonium-241 (the half-life of 241Pu is 14.4 years). The half-life of americium-241 is 432.2 years, during the decay an alpha particle is released. Americium dissolves in water much better than plutonium, therefore it has a greater migration ability. Accumulates up to 99% in the surface layers of the soil, 10% americium is in dissolved form and is easily absorbed by plants. Concentrates in humans in the skeleton, liver, kidneys. The elimination half-life from the skeleton is up to 30 years, from the liver - up to 5 years.