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(β −)
235 Np ()
239 Pu () |
| Spin and parity of the nucleus |
7/2 −
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| Decay channel
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Decay energy
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|---|
| α decay |
4.6783 (7) MeV |
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| 20 Ne, 25 Ne, 28 Mg |
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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 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 directions:
Forced division
In the fission products of uranium-235, about 300 isotopes of various elements were found: from = 30 (zinc) to Z = 64 (gadolinium). The curve of the dependence of the relative yield of isotopes formed during irradiation of uranium-235 with slow neutrons on the mass number is symmetric and resembles the letter "M" in shape. 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) is less likely 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 with an increase in the excitation energy of a fissile nucleus, and at a neutron energy of more than 100 MeV, the mass distribution of fission fragments has one maximum, corresponding to symmetric nuclear fission. The fragments formed during the fission of a 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 possible in the interaction of neutrons with nuclei; it is this that 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 formed during the decay of the 235 U nucleus, subject to interaction with another 235 U nucleus, can cause a new decay act, this phenomenon is called nuclear fission chain reaction.
Hypothetically, the number of second-generation neutrons (after the second stage of nuclear decay) can exceed 3² = 9. With each subsequent stage of the fission reaction, the number of produced neutrons can grow like an avalanche. In real conditions, free neutrons may not generate a new fission act, leaving the sample before the capture of 235 U, or being captured both by the 235 U isotope itself, transforming it into 236 U, and by other materials (for example, 238 U, or the resulting nuclear fission fragments, such as 149 Sm or 135 Xe).
Under real conditions, it is not so easy to reach the critical state of uranium, since a number of factors influence the course of the reaction. For example, natural uranium consists of only 0.72% of 235 U, 99.2745% is 238 U, which absorbs neutrons formed during the fission of 235 U nuclei. fades out quickly. A sustained fission chain reaction can be carried out in several main ways:
- Increase the sample volume (for uranium separated from ore, it is possible to achieve a critical mass by increasing the volume);
- Perform isotope separation by increasing the concentration of 235 U in the sample;
- Reduce the loss of free neutrons through the sample surface by using various types of reflectors;
- Use a neutron moderator to increase the concentration of thermal neutrons.
Isomers
- Excess mass: 40 920.6 (1.8) keV
- Excitation energy: 76.5 (4) eV
- Half-life: 26 min
- Spin and parity of the core: 1/2 +
The decay of the isomeric state is carried out by the isomeric transition to the ground state.
Application
- Uranium-235 is used as fuel for nuclear reactors in which managed chain nuclear fission reaction;
- Highly enriched uranium is used to create nuclear weapons. In this case, to release a large amount of energy (explosion), uncontrollable nuclear chain reaction.
see also
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Notes (edit)
- G. Audi, A.H. Wapstra, and C. Thibault (2003). "". Nuclear Physics A 729
: 337-676. DOI: 10.1016 / j.nuclphysa.2003.11.003. Bibcode:.
- 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:.
- Hoffman K.- 2nd ed. erased. - L.: Chemistry, 1987 .-- P. 130 .-- 232 p. - 50,000 copies.
-
- Fialkov Yu. Ya. Application of isotopes in chemistry and chemical industry. - Kiev: Tekhnika, 1975 .-- P. 87 .-- 240 p. - 2,000 copies
- ... Kaye & Laby Online. ...
- Bartolomey G.G., Baibakov V.D., Alkhutov M.S., Bat G.A. Fundamentals of the theory and methods of calculating nuclear power reactors. - M .: Energoatomizdat, 1982 .-- S. 512.
Easier:
uranium-234
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Uranium-235 is
isotope of uranium
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Heavier:
uranium-236
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| Isotopes of elements Nuclide table |
Excerpt from Uranus-235
Miloradovich, who said that he did not want to know anything about the economic affairs of the detachment, which could never be found when it was needed, "chevalier sans peur et sans reproche" ["a knight without fear and reproach"], as he called himself , and a hunter before talking with the French, sent envoys, demanding surrender, and wasted time and did not what he was ordered to.
“I give you guys this column,” he said, approaching the troops and pointing the cavalrymen at the French. And cavalrymen on skinny, skinned, barely moving horses, urging them on with spurs and sabers, trotted, after strong stresses, drove up to the presented column, that is, to the crowd of frostbitten, numb and hungry French; and the donated column threw down its weapons and surrendered, which it had long wanted.
Near Krasnoye, they took twenty-six thousand prisoners, hundreds of cannons, some kind of stick, which was called a marshal's baton, and argued about who distinguished himself there, and were pleased with this, but very much regretted that they did not take Napoleon or at least some hero, Marshal, and reproached each other for this, and especially Kutuzov.
These people, carried away by their passions, were blind executors of only the saddest law of necessity; but they considered themselves heroes and imagined that what they did was the most worthy and noble deed. They accused Kutuzov and said that from the very beginning of the campaign he had prevented them from defeating Napoleon, that he only thought about satisfying his passions and did not want to leave the Linen Mills, because he was at peace there; that he stopped movement near Krasnoye only because, having learned about Napoleon's presence, he was completely lost; that it can be assumed that he is in a conspiracy with Napoleon, that he has been bribed by him, [Wilson's notes. (Leo Tolstoy's note.)] Etc., etc.
Not only did his contemporaries, carried away by passions, say so, - posterity and history recognized Napoleon as grand, and Kutuzov: foreigners - a cunning, depraved, weak court old man; Russians - somehow indefinite - some kind of doll, useful only by its Russian name ... In the 12th and 13th years, Kutuzov was directly accused of mistakes. The sovereign was dissatisfied with him. And in a story recently written at the highest command, it is said that Kutuzov was a cunning court liar who feared the name of Napoleon and, by his mistakes at Krasnoye and Berezina, deprived the Russian troops of glory - a complete victory over the French. [The story of Bogdanovich in 1812: a characteristic of Kutuzov and a discussion about the unsatisfactory results of the Krasnensky battles. (Leo Tolstoy's note.)]
This is not the fate 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 the higher laws.
For Russian historians - it is strange and scary to say - Napoleon is the most insignificant instrument of history - never and nowhere, even in exile, who did not show human dignity - Napoleon is an object of admiration and delight; he is grand. Kutuzov, the person who, from the beginning to the end of his activity in 1812, from Borodino to Vilna, never once by any action, not betrayed himself by a word, is an extraordinary example of selflessness and consciousness in the present of the future meaning of an event in history, - Kutuzov seems to them something vague and pathetic, and, speaking of Kutuzov and the 12th year, they always seem to be a little ashamed.
And yet it is difficult to imagine a historical person whose activity would be so invariably constantly directed towards one and the same goal. It is difficult to imagine a goal more worthy and more in line with the will of the entire people. It is even more difficult to find another example in history, where the goal set by a historical person would be as completely achieved as the goal towards which all of Kutuzov's activities were directed in 1812.
Kutuzov never spoke 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 committed: he did not say anything about himself at all, did not play any role, always seemed to be the simplest and most 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 replied: "I will not leave Moscow without a battle," despite the fact that Moscow had already been abandoned. When Arakcheev, who came to him from the sovereign, said that Ermolov should be appointed chief of artillery, Kutuzov replied: “Yes, I myself have just said that,” although he said something completely different in a minute. What business was it to him, who alone then understood the whole tremendous meaning of the event, among the stupid crowd that surrounded him, what did he care whether Count Rostopchin took the disaster of the capital to himself or to him? Still less could he be interested in who was appointed chief of artillery.
Not only in these cases, but incessantly this old man, who has come down to the experience of life to the conviction that the thoughts and words that serve as their expression are not the movers of people, spoke words completely meaningless - the first ones that occurred to him.
But this very man, who so neglected his words, never once in all his activity said a single word that would not agree with the only goal to which he was going during the whole war. Obviously, involuntarily, with a heavy confidence that they would not understand him, he repeatedly expressed his thought in a wide variety of 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 repeated this both orally, 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 Lauriston's proposal for peace, he replied that there can be no peace, because such is the will of the people; he alone, during the retreat of the French, said that all our maneuvers were unnecessary, that everything would turn out by itself better than we wish, that the enemy should be given a golden bridge, that neither Tarutinskoe, nor Vyazemskoe, nor Krasnenskoe battles were needed, that with what one day he must come to the border, that for ten Frenchmen he will not give up one Russian.
And he is alone, this court man, as we are portrayed, a man who lies to Arakcheev in order to please the sovereign - he alone, this court man, in Vilna, thus deserving the sovereign's disfavor, says that a further war abroad is harmful and useless.
But words alone would not prove that he then understood the meaning of the event. His actions were all without the slightest deviation, all were directed towards the same goal, expressed in three actions: 1) to exert all his forces to confront the French, 2) to defeat them, and 3) to expel them from Russia, making it as easy as possible, the calamities of the people and the troops.
He, the procrastinator of Kutuzov, whose motto is patience and time, the enemy of decisive actions, he gives the Battle of Borodino, clothe preparations for it in unparalleled solemnity. He, the same Kutuzov, who in the battle of Austerlitz, before starting it, 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 a battle won, the army must retreat , he alone, in opposition to everyone, until his death asserts that the Battle of Borodino is a victory. He alone during the entire retreat insists not to give battles, which are now useless, not to start a new war and not to cross the borders of Russia.
Now it is easy to understand the meaning of the event, if only not to apply to the activities of the masses the goals that were in the head of a dozen people, it is easy, since the entire event with its consequences lies before us.
But how then could this old man, alone, contrary to the opinion of everyone, have guessed, so correctly guessed then the meaning of the national meaning of the event, that he never once betrayed it in all his activities?
The source of this extraordinary power of insight into the meaning of occurring phenomena lay in that popular feeling, which he carried in 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 in disfavor to choose him against the will of the tsar as a representative 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.
By March 1939, teams of scientists working in France and America had proven that an average of two to four free neutrons were released for a self-sustaining chain reaction for every fission of a uranium nucleus. The growing fears about the possibility of creating an atomic bomb, however, were quickly dispelled.
Bohr decided not to waste time. The physics of fission, like any other new direction in science, undoubtedly provided an immense field for activity. And, since Princeton could work with no less success than Copenhagen, Bohr approached Wheeler with an offer of cooperation. They set about further development of the theory of nuclear fission, relying on new experimental data. They carried out the experiments with a device that was hastily assembled right there, in Princeton, in the attic of the Palmer laboratory. The results were puzzling at first.
The apparatus mentioned above was needed to study changes in the intensity of fission of a uranium nucleus under the influence of neutrons, each time carrying different amounts of energy. It was found that the greater this energy, the more intense the fission occurs, and with its decrease, the fission intensity, accordingly, also decreases. Such data were quite expected. However, it soon became clear that with a sufficient decrease in the neutron energy, the intensity of nuclear fission increases again.
Placzek, who had previously led Frisch, who worked in Copenhagen, to seek reliable confirmation of nuclear fission, quite unexpectedly found himself in Princeton. "What the hell is this: why the response is the same for both fast and slow impacts ?!" - he was indignant, sitting at breakfast with Rosenfeld and Bohr.
Returning soon to his office, Niels Bohr already knew the answer to this question. Apparently, the reason for the high intensity of nuclear fission at low energies of acting neutrons is the rare isotope uranium-235 (U 235), which makes up a negligible percentage of the total amount of this element found in nature. Bohr and Wheeler now began to elaborate on this hypothesis. And in the new theory, two fundamental factors were established.
In the U 235 isotope, the balance between the repulsive force of protons in the atomic nucleus and the surface tension that keeps the nucleus from decay is much more fragile than in the U 238 isotope. Uranium-238's three additional neutrons stabilize the nucleus and increase the energy barrier that must be overcome to trigger the decay reaction. Consequently, for the splitting of such a nucleus, faster neutrons with higher energy are needed.
The second of the factors mentioned was the complex composition of the nucleus itself. For him, an equal number of protons and neutrons is more favorable, which is explained by the quantum nature of their subatomic constituents. Having received an additional neutron, U 235 turns into U 236, in the nucleus of which there are 92 protons and 144 neutrons, that is, an even number of both nucleons. When U 238 receives an extra neutron, the isotope U 239 is formed with an odd number of neutrons in the nucleus. Uranium-235 "assimilates" an additional neutron and reacts with it much easier than uranium-238.
The combination of the above two factors sufficiently explains the significant difference in the behavior of the two isotopes of uranium. Fission of the stable U 238 nucleus requires fast neutrons, while the much less stable U 235 nucleus can be fissioned slowly. Thus, if a bomb is made consisting of a mixture of U 235 and U 238, the action of which is based on the fission of uranium-235 under the influence of slow neutrons, then the chain reaction in it will proceed slowly. Then it will die out, and the bomb will never detonate.
Now the chances of creating a bomb in the near future, although not completely disappeared, have significantly decreased. Of course, we must not forget about the words of Bohr, which he repeated many times during discussions with colleagues in April 1939: then he said that to make a bomb can provided that it is made on the basis of pure uranium-235. However, U 235 is a rare isotope and its share in relation to natural uranium is 1: 140, that is, an insignificant 0.7%. In addition, U 235 and U 238 are identical in chemical properties, and therefore cannot be separated using a chemical reaction. This is possible only with the use of special physical methods that make it possible to separate isotopes from each other, using an almost imperceptible difference in their mass. At the same time, such work on the scale required to create an atomic bomb required unreasonably large efforts - at the then level of development, it required several tons of uranium-235.
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), othenite (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, silvery-white metal. It is ductile, malleable, and amenable to polishing. In the air, the metal oxidizes and ignites in the crushed state. Relatively poorly conductive. 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 recently discovered planet Uranus, the metal itself was isolated in 1841 by the French chemist Eugene-Melchior Peligot by reduction from uranium tetrachloride (UCl 4) with potassium.
Radioactivity
The creation of the periodic table by the Russian chemist Dmitry 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 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 a series of actinium decay.
Opening a chain reaction
The chemical element uranium became the subject of widespread interest and intense study after German chemists Otto Hahn and Fritz Strassmann discovered nuclear fission in it at the end of 1938 when bombarded with slow neutrons. In early 1939, an American physicist of Italian origin, Enrico Fermi, suggested that among the fission products of an atom there may be elementary particles capable of generating a chain reaction. In 1939, 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 when an atom fissions. 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 hostilities (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 oxide UO 2, tetrahalides such as UCl 4, and green aqueous ion U 4+) and +6 (as in oxide UO 3, hexafluoride UF 6 and uranyl ion UO 2 2+). 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 states +3 and +5, but they are unstable. Red U 3+ is slowly oxidized in oxygen-free water. 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 a 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. At the same time, after absorption and negative beta decay, uranium-238 turns into a synthetic element plutonium, which splits under the action of slow neutrons. Therefore, natural uranium can be used in converting reactors and breeders, in which fission is supported by the rare 235 U and plutonium is produced simultaneously with the transmutation of 238 U. The fissile 233 U can be synthesized from the thorium-232 isotope widespread in nature for use as a nuclear fuel. Uranium is also important as the primary material from which synthetic transuranium elements are derived.
Other uses of uranium
Compounds of a 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 gaseous diffusion and gas centrifuge methods for producing enriched uranium.
Organometallic compounds are an interesting and important group of compounds in which metal-carbon bonds link metal to organic groups. Uranocene is an organo-uranic compound U (C 8 H 8) 2 in which a uranium atom is sandwiched between two layers of organic rings bonded to cyclooctatetraene C 8 H 8. Its discovery in 1968 opened up a new field of organometallic chemistry.
Depleted natural uranium is used as a means of radiation protection, ballast, armor-piercing shells and tank armor.
Processing
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 position 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%) 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 it is absorbed, 238 U forms 239 U, which ultimately decays into 239 Pu, a fissile material of great importance for nuclear power and nuclear weapons. Another fissile isotope, 233 U, can be produced by neutron irradiation of 232 Th.
Crystalline forms
The characteristics of uranium determine its reaction 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 rarely occurs 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 the 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, and β - tetragonal. The α phase consists of layers of atoms in a highly symmetric orthorhombic structure. This anisotropic distorted structure prevents the alloying metal atoms from replacing uranium atoms or occupying the space between them in the crystal lattice. It was found that solid solutions form only molybdenum and niobium.
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 × 10 9 tonnes of this chemical element. Uranium is an important component of over 150 different minerals and a minor component of another 50. Primary minerals found in magmatic hydrothermal veins and in pegmatites include uraninite and 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 of uranium mines are autunite (hydrated calcium uranyl phosphate), tobernite (hydrated copper uranyl phosphate), coffinite (hydrated black uranium silicate) and carnotite (hydrated potassium uranyl vanadate).
It is estimated that over 90% of known low-cost uranium reserves are found in Australia, Kazakhstan, Canada, Russia, South Africa, Niger, Namibia, Brazil, PRC, Mongolia and Uzbekistan. Large deposits are found in the conglomerate rock formations of Lake Elliot, located north of Lake Huron in Ontario, Canada, and in the South African Witwatersrand gold mine. Sand formations on the Colorado Plateau and in the Wyoming Basin of the western United States also contain significant reserves of uranium.
Mining
Uranium ores are found both in near-surface and deep (300-1200 m) sediments. Under the ground, the seam thickness reaches 30 m. As in the case of other metal ores, uranium is mined on the surface with large earth-moving equipment, and deep sediments are mined using traditional vertical and inclined mine methods. World production of uranium concentrate in 2013 amounted to 70 thousand tons. The most productive uranium mines are located in Kazakhstan (32% of all production), Canada, Australia, Niger, Namibia, Uzbekistan and Russia.
Uranium ores usually contain only small amounts of uranium-bearing minerals and cannot be smelted by direct pyrometallurgical methods. Instead, hydrometallurgical procedures must be used to extract and purify uranium. Increasing the concentration significantly 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 manual sorting, are applicable. With a few exceptions, these methods result in significant losses of uranium.
Burning
The hydrometallurgical treatment of uranium ores is often preceded by a high-temperature calcination stage. Roasting dehydrates clay, removes carbonaceous materials, oxidizes sulfur compounds to harmless sulfates, and oxidizes any other reducing agents that might interfere with subsequent processing.
Leaching
Uranium is extracted from roasted ores with both acidic and alkaline aqueous solutions. For the successful functioning of all leaching systems, a chemical element must either initially be present in a more stable 6-valent form, or be oxidized to this state during processing.
Acid leaching is usually carried out by stirring a mixture of ore and lixiviant for 4-48 hours at ambient temperature. Sulfuric acid is used except in special circumstances. It is fed in quantities sufficient to produce the final liquor at a pH of 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+). Typically, 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 uranyl sulfate complex anion 4-.
Ore containing significant amounts of basic minerals such as calcite or dolomite is leached with 0.5-1 molar sodium carbonate solution. Although various reagents have been studied and tested, oxygen is the main oxidizing agent for uranium. Typically, the ore is leached in air at atmospheric pressure and at a temperature of 75-80 ° C for a period of time, which depends on the specific chemical composition. Alkali reacts with uranium to form the 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 sludges 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 ion exchange resin leaching solutions. These special resins, characterized by their sorption and elution kinetics, particle size, stability and hydraulic properties, can be used in various processing technologies, for example in fixed and moving beds, ion exchange resin in basket and continuous pulp. Usually, solutions of sodium chloride and ammonia or nitrates are used to elute sorbed uranium.
Uranium can be isolated from acidic ore liquors by solvent extraction. The industry uses alkyl phosphoric acids, as well as secondary and tertiary alkyl amines. As a rule, solvent extraction is preferred over ion exchange methods for acidic filtrates containing more than 1 g / L of uranium. However, this method is not applicable to carbonate leaching.
Then uranium is purified by dissolving in nitric acid with the formation of uranyl nitrate, extracted, crystallized and calcined with the formation of trioxide UO 3. Reduced UO2 dioxide reacts with hydrogen fluoride to form UF4 thetafluoride, from which uranium metal is reduced with 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.
The main method for extracting uranium-235 from natural uranium has become the gaseous diffusion method. Soviet scientists Kikoin, Sobolev and Smorodinsky developed the theory of the gas diffusion process. The gas diffusion method is based on a slight difference in the speed of movement of heavy uranium-238 nuclei and less heavy uranium-235 nuclei when the gaseous uranium compound passes through special porous baffles. With a single passage of gas, it is possible to increase the content of the uranium-235 isotope by only 0.2%. To enrich uranium with the 235 isotope to 90–94 percent, which is exactly what is required for a warhead, it is necessary to pump the gas through a diffusion stage with a porous baffle several thousand times.
The development and manufacture of porous partitions turned out to be a very difficult problem; both the yield of finished products and the consumption of electricity for pumping gas depended on their quality. It was not easy to design and manufacture reliable and simple compressors for pumping gas with a high degree of tightness so that toxic gas product does not enter the production area.
The construction of the gaseous diffusion plant began in 1946. At the beginning of construction, manual labor and horse-drawn traction were also used here, only in 1948 the first excavator arrived here. The work was carried out around the clock. The design of the plant and its installations was extremely complex. The main building of the plant had an area of over 100 thousand square meters. During the adjustment of the systems, there were numerous shutdowns. The compressor supplier very quickly carried out the reconstruction and even the replacement of equipment, these works were under the personal supervision of Beria and Stalin. After the reconstruction, several thousand diffusion machines of four modifications were installed at the plant.
Despite all the difficulties, the business was progressing and in 1948 uranium-235 was obtained with an enrichment of 75%. It wasn't enough. Then they made an interim decision. Uranium-235 began to be sent for further enrichment by the electromagnetic method, up to 90 percent or more.
In 1950, the gaseous diffusion plant increased the enrichment to 90% and reached the design capacity; in 1951, the uranium enrichment exceeded 90%.
The basis of the plant for electromagnetic isotope separation was a huge electromagnetic installation, equipped with special chambers made of scarce brass. The installation took a long time to set up, and in 1949 it produced uranium with an enrichment of more than 90%. Later the plant expanded.
Thus, the problem of the production of two types of nuclear explosives was solved: plutonium and uranium-235 in sufficient quantities for the manufacture of Soviet nuclear weapons.
Plutonium is a man-made element. Before the atomic era in nature there were only its "traces" - several tens of kilograms in the entire thickness of the earth's crust. Now - hundreds of tons, and not in the entire earth's crust, but in bombs and warehouses, plus tons scattered over the surface of the planet.
In just one year, all reactors in the world produce 10 thousand tons of spent nuclear fuel, which contains 100 tons of plutonium, that is, each ton of spent nuclear fuel contains ~ 10 kg of plutonium (for comparison, the bomb dropped on Nagasaki contained only 6.2 kg ).
Although reactor plutonium, isolated during the reprocessing of spent nuclear fuel, is not weapon-grade, it is still possible to make a bomb from it. The world is already full of allocated plutonium for making bombs. There is a lot of it: in deployed weapons systems, in warheads intended for dismantling, in waste during the cleaning of nuclear weapons complexes, in warehouses at processing plants.
Fissile, that is, weapons-grade, is the isotope - plutonium-239. For its production, in addition to enriched uranium (fuel), unenriched, natural, uranium ("raw material") was placed in a military reactor in the form of metal blocks enclosed in a sealed aluminum shell. During the fission reaction in the reactor core, a large flux of neutrons occurs and the uranium blocks are irradiated with these neutrons (hence the term "irradiated uranium" or irradiated nuclear fuel).
When neutrons are captured, the nuclei of uranium atoms are converted into plutonium nuclei, therefore, inside the blocks, non-fissile uranium-238 gradually turned into fissile (weapon-grade) plutonium-239. During the holding time in the reactor (3-6 months), several hundred grams of uranium-238 were transformed from each ton of natural uranium into plutonium-239.
