What is the mass of 1 ku of uranium 238. Uranium half-life: main characteristics and applications

Uranus is a naturally occurring element that finds application, among other things, in nuclear power engineering. Natural uranium consists mainly of a mixture of three isotopes: 238U, 235U and 234U.

Depleted uranium (DU) - is a by-product of the uranium enrichment process (i.e., increasing the content of the fissile isotope 235U in it) in nuclear power; the radioactive isotope 234U is almost completely removed from it and 235U is removed by two-thirds. Thus, DU consists almost entirely of 238U, and its radioactivity is about 60% of that of natural uranium. The DU may also contain a trace amount of other radioactive isotopes introduced during processing. Chemically, physically and toxically, DU behaves in the same way as natural uranium in the metallic state. Small particles of both metals easily ignite, forming oxides.

Application of depleted uranium. For peaceful purposes, DU is used, in particular, in the manufacture of aircraft counterweights and anti-radiation screens for medical radiotherapy equipment, and in the transportation of radioactive isotopes. Due to its high density and infusibility, as well as the availability of DU, it is used in heavy tank armor, anti-tank ammunition, rockets and projectiles. Weapons containing DU are considered conventional weapons and are freely used by the armed forces.

Issues raised by the use of depleted uranium . From a fired munition, depleted uranium is released as fine particles or dust, which can be inhaled or ingested, or remain in the environment. There is a possibility that the use of DU weapons affects the health of people living in conflict areas in the Persian Gulf and the Balkans. Some believe that the "Gulf War Syndrome" is associated with exposure to depleted uranium, but a causal relationship has not yet been established. DU has been released into the environment as a result of air crashes (eg: Amsterdam, Netherlands, 1992; Stansted, United Kingdom, January 2000), causing concern to governments and non-governmental organizations.

Depleted uranium and human health. The impact of DU on human health is different depending on the chemical form in which it enters the body, and can be caused by both chemical and radiological mechanisms. Little is known about how uranium affects human health and the environment. However, since uranium and DU are essentially the same, except for the composition of the radioactive components, scientific research on natural uranium is applicable to DU as well. With regard to the radiation impact of DU, the picture is further complicated by the fact that most of the data relate to the effects of natural and enriched uranium on the human body. The health impact depends on how the exposure occurred and the extent of exposure (inhalation, ingestion, contact or wound) and on the characteristics of the DU (particle size and solubility). The likelihood of detecting a potential impact depends on the setting (military, civilian life, work environment).

Irradiation types . Under normal human consumption of food, air and water, there is an average of about 90 micrograms (mcg) of uranium present: about 66% in the skeleton, 16% in the liver, 8% in the kidneys, and 10% in other tissues. External exposure occurs when close to a metal DU (for example, when working in an ammunition depot or while in a vehicle with ammunition or armor in which DU is present) or through contact with dust or fragments formed after an explosion or fall. Exposure received only externally (i.e., not by ingestion, not through the respiratory tract, and not through the skin) results in consequences of a purely radiological nature. Internal exposure occurs as a result of DU entering the body through ingestion or inhalation. In the army, radiation also occurs through wounds formed by contact with shells or armor in which DU is present.

Absorption of uranium in the body. Most (over 95%) of the uranium that enters the body is not absorbed, but is removed with feces. Of the part of the uranium that is absorbed by the blood, approximately 67% will be filtered out by the kidneys within a day and removed in the urine. Uranium is transported to the kidneys, bone tissue and liver. It is estimated that it takes 180 to 360 days to eliminate half of this uranium in the urine.

Health Hazard:

Chemical toxicity: Uranium causes kidney damage in experimental animals, and some studies indicate that long-term exposure can lead to impaired renal function in humans. Observed types of disorders: nodular formations on the surface of the kidney, damage to the tubular epithelium and an increase in the content of glucose and protein in the urine.

Radiological toxicity: DU decays primarily by emitting alpha particles, which do not penetrate the outer layers of the skin, but can affect the body's internal cells (more susceptible to the ionizing effects of alpha radiation) when DU is ingested or inhaled. Therefore, alpha and beta irradiation by inhalation of insoluble DU particles can damage lung tissue and increase the risk of lung cancer. Similarly, it is assumed that the absorption of DU in the blood and its accumulation in other organs, in particular in the skeleton, creates an additional risk of cancer of these organs, depending on the degree of radiation exposure. It is believed, however, that at a low degree of exposure, the risk of cancer is very low.

As part of the limited epidemiological studies performed to date on internal exposure to DU particles by ingestion, inhalation, or through skin lesions or wounds, as well as surveys of people whose occupations come into contact with natural or enriched uranium, no negative health effects were found.

Depleted uranium in the environment. In arid regions, most of the DU remains on the surface in the form of dust. In more rainy areas, DU penetrates the soil more easily. Cultivation of contaminated soil and consumption of contaminated water and food may create health risks, but they are likely to be minor. The main health hazard will be chemical toxicity rather than radiation exposure. The risk of exposure to depleted uranium from contaminated food and water when returning to normal life in a war zone appears to be greater for children than for adults, as children tend to put things from hand to mouth due to their curiosity, which can lead to to ingestion of a large amount of DU from contaminated soil.

Standards. WHO has regulations for uranium that apply to DU. Currently these standards are:

"Guidelines for the quality control of drinking water": 2 μg / l - an indicator that is considered safe based on data on subclinical renal changes given in epidemiological studies (WHO, 1998);

Acceptable Daily Intake (ADI) for ingestion of uranium by mouth: 0.6 µg per kilogram of body weight per day (WHO, 1998);

limit norms of ionizing radiation: 1 mSv per year for the general population and 20 mSv on average per year for five years for persons working in a radiation environment (Basic Safety Standards, 1996).

isotopes uranium - varieties of atoms (and nuclei) of the chemical element uranium, having a different content of neutrons in the nucleus. At the moment, 26 isotopes of uranium and 6 more excited isomeric states of some of its nuclides are known. There are three isotopes of uranium in nature: 234U (isotope abundance 0.0055%), 235U (0.7200%), 238U (99.2745%).

The nuclides 235U and 238U are the founders of the radioactive series - the actinium series and the radium series, respectively. The nuclide 235U is used as a fuel in nuclear reactors, as well as in nuclear weapons (due to the fact that a self-sustaining nuclear chain reaction is possible in it). The nuclide 238U is used to produce plutonium-239, which is also extremely important both as a fuel for nuclear reactors and in the production of nuclear weapons. Characteristics of uranium isotopes are given in Table 1.

Table 1 - Characteristics of uranium isotopes

Note:

Isotope abundances are given for most natural samples. For other sources, the values ​​may vary greatly.

The indices "m", "n", "p" (next to the symbol) denote the excited isomeric states of the nuclide.

Values ​​marked with a hash (#) are not derived from experimental data alone, but are (at least partially) estimated from systematic trends in neighboring nuclides (with the same Z and N ratios). Uncertainly determined values ​​of the spin and/or its parity are enclosed in brackets.

Nuclear fission is the splitting of a heavy atom into two fragments of approximately equal mass, accompanied by the release of a large amount of energy.

The discovery of nuclear fission began a new era - the "atomic age". The potential of its possible use and the ratio of risk to benefit from its use have not only generated many sociological, political, economic and scientific achievements, but also serious problems. Even from a purely scientific point of view, the process of nuclear fission has created a large number of puzzles and complications, and its full theoretical explanation is a matter of the future.

Sharing is profitable

The binding energies (per nucleon) differ for different nuclei. Heavier ones have lower binding energies than those located in the middle of the periodic table.

This means that for heavy nuclei with an atomic number greater than 100, it is advantageous to divide into two smaller fragments, thereby releasing energy, which is converted into the kinetic energy of the fragments. This process is called splitting

According to the stability curve, which shows the dependence of the number of protons on the number of neutrons for stable nuclides, heavier nuclei prefer more neutrons (compared to the number of protons) than lighter ones. This suggests that along with the splitting process, some "spare" neutrons will be emitted. In addition, they will also take on some of the released energy. The study of nuclear fission of the uranium atom showed that 3-4 neutrons are released: 238 U → 145 La + 90 Br + 3n.

The atomic number (and atomic mass) of the fragment is not equal to half the atomic mass of the parent. The difference between the masses of atoms formed as a result of splitting is usually about 50. True, the reason for this is not yet entirely clear.

The binding energies of 238 U, 145 La, and 90 Br are 1803, 1198, and 763 MeV, respectively. This means that as a result of this reaction, the fission energy of the uranium nucleus is released, equal to 1198 + 763-1803 = 158 MeV.

Spontaneous division

The processes of spontaneous splitting are known in nature, but they are very rare. The average lifetime of this process is about 10 17 years, and, for example, the average lifetime of alpha decay of the same radionuclide is about 10 11 years.

The reason for this is that in order to split into two parts, the nucleus must first be deformed (stretched) into an ellipsoidal shape, and then, before finally splitting into two fragments, form a “neck” in the middle.

Potential Barrier

In the deformed state, two forces act on the core. One is the increased surface energy (the surface tension of a liquid drop explains its spherical shape), and the other is the Coulomb repulsion between fission fragments. Together they produce a potential barrier.

As in the case of alpha decay, in order for the spontaneous fission of the uranium atom nucleus to occur, the fragments must overcome this barrier using quantum tunneling. The barrier is about 6 MeV, as in the case of alpha decay, but the probability of tunneling an alpha particle is much greater than that of a much heavier atom fission product.

forced splitting

Much more likely is the induced fission of the uranium nucleus. In this case, the parent nucleus is irradiated with neutrons. If the parent absorbs it, they bind, releasing binding energy in the form of vibrational energy that can exceed the 6 MeV required to overcome the potential barrier.

Where the energy of the additional neutron is insufficient to overcome the potential barrier, the incident neutron must have a minimum kinetic energy in order to be able to induce the splitting of an atom. In the case of 238 U, the binding energy of additional neutrons is about 1 MeV short. This means that fission of the uranium nucleus is induced only by a neutron with a kinetic energy greater than 1 MeV. On the other hand, the 235 U isotope has one unpaired neutron. When the nucleus absorbs an additional one, it forms a pair with it, and as a result of this pairing, additional binding energy appears. This is enough to release the amount of energy necessary for the nucleus to overcome the potential barrier and the isotope fission occurs upon collision with any neutron.

beta decay

Even though the fission reaction emits three or four neutrons, the fragments still contain more neutrons than their stable isobars. This means that cleavage fragments are generally unstable against beta decay.

For example, when uranium 238U fission occurs, the stable isobar with A = 145 is neodymium 145Nd, which means that the lanthanum 145La fragment decays in three steps, each time emitting an electron and an antineutrino, until a stable nuclide is formed. The stable isobar with A = 90 is zirconium 90 Zr; therefore, the bromine 90 Br splitting fragment decomposes in five stages of the β-decay chain.

These β-decay chains release additional energy, which is almost all carried away by electrons and antineutrinos.

Nuclear reactions: fission of uranium nuclei

Direct emission of a neutron from a nuclide with too many of them to ensure the stability of the nucleus is unlikely. The point here is that there is no Coulomb repulsion, and so the surface energy tends to keep the neutron in bond with the parent. However, this sometimes happens. For example, a 90 Br fission fragment in the first beta decay stage produces krypton-90, which can be in an excited state with enough energy to overcome the surface energy. In this case, the emission of neutrons can occur directly with the formation of krypton-89. still unstable with respect to β decay until converted to stable yttrium-89, so that krypton-89 decays in three steps.

Fission of uranium nuclei: a chain reaction

The neutrons emitted in the fission reaction can be absorbed by another parent nucleus, which then itself undergoes induced fission. In the case of uranium-238, the three neutrons that are produced come out with an energy of less than 1 MeV (the energy released during the fission of the uranium nucleus - 158 MeV - is mainly converted into the kinetic energy of the fission fragments), so they cannot cause further fission of this nuclide. Nevertheless, at a significant concentration of the rare isotope 235 U, these free neutrons can be captured by 235 U nuclei, which can indeed cause fission, since in this case there is no energy threshold below which fission is not induced.

This is the principle of a chain reaction.

Types of nuclear reactions

Let k be the number of neutrons produced in a sample of fissile material in stage n of this chain, divided by the number of neutrons produced in stage n - 1. This number will depend on how many neutrons produced in stage n - 1 are absorbed by the nucleus, which may be forced to divide.

If k< 1, то цепная реакция просто выдохнется и процесс остановится очень быстро. Именно это и происходит в природной в которой концентрация 235 U настолько мала, что вероятность поглощения одного из нейтронов этим изотопом крайне ничтожна.

If k > 1, then the chain reaction will grow until all the fissile material has been used. This is achieved by enriching natural ore to obtain a sufficiently large concentration of uranium-235. For a spherical sample, the value of k increases with an increase in the neutron absorption probability, which depends on the radius of the sphere. Therefore, the mass U must exceed a certain amount in order for the fission of uranium nuclei (chain reaction) to occur.

If k = 1, then a controlled reaction takes place. This is used in a process controlled by distributing cadmium or boron rods among the uranium, which absorb most of the neutrons (these elements have the ability to capture neutrons). The fission of the uranium nucleus is automatically controlled by moving the rods in such a way that the value of k remains equal to one.

Application

Although uranium-238 cannot be used as a primary fissile material, due to the high energy of the neutrons required for its fission, it has an important place in the nuclear industry.

Having a high density and atomic weight, U-238 is suitable for making reflector charge shells from it in fusion and fission devices. The fact that it is fissile with fast neutrons increases the energy yield of the charge: indirectly, by multiplying reflected neutrons; directly during the fission of shell nuclei by fast neutrons (during synthesis). Approximately 40% of the neutrons produced during fission and all fusion neutrons have sufficient energies for U-238 fission.

U-238 has a spontaneous fission rate 35 times higher than U-235, 5.51 fiss/s*kg. This makes it impossible to use it as a shell of a reflector charge in cannon bombs, because its suitable mass (200-300 kg) will create too high a neutron background.

Pure U-238 has a specific radioactivity of 0.333 microcurie/g.

An important area of ​​application for this uranium isotope is the production of plutonium-239. Plutonium is formed in the course of several reactions that begin after the capture of a neutron by a U-238 atom. Any reactor fuel containing natural or partially enriched uranium in the 235th isotope contains a certain proportion of plutonium after the end of the fuel cycle.

Decay chain of uranium-238

The isotope uranium-238, it is more than 99% in natural uranium. This isotope is also the most stable; thermal neutrons cannot split its nucleus. In order to split 238U, the neutron needs an additional kinetic energy of 1.4 MeV. A nuclear reactor made of pure uranium-238 will not work under any circumstances.

An atom of uranium-238, in whose nucleus the protons and neutrons are barely held together by cohesive forces. From time to time, a compact group of four particles escapes from it: two protons and two neutrons (b-particle). Uranium-238 is thus converted into thorium-234, which contains 90 protons and 144 neutrons in its nucleus. But thorium-234 is also unstable. Its transformation, however, is not the same as in the previous case: one of its neutrons turns into a proton, and thorium-234 turns into protactinium-234, the nucleus of which contains 91 protons and 143 neutrons. This metamorphosis, which took place in the nucleus, also affects the electrons moving in their orbits: one of them becomes unpaired and flies out of the atom. Protactinium is very unstable and takes very little time to transform. This is followed by other transformations, accompanied by radiation, and this whole chain, in the end, ends with a stable lead nuclide (see Figure No. 7, Appendix B).

The most important circumstance for nuclear energy is that the most common uranium isotope, 238U, is also a potential source of nuclear fuel. Both Szilard and Fermi were right in assuming that the absorption of neutrons by uranium would lead to the formation of new elements. Indeed, in a collision with a thermal neutron, uranium-238 does not fission; instead, the nucleus absorbs the neutron. On average, in 23.5 minutes, one of the neutrons in the nucleus turns into a proton (with the emission of an electron, the reaction in - decay), and the nucleus of uranium-239 becomes the nucleus of neptunium-239 (239Np). After 2.4 days, the second β-decay occurs and plutonium-239 (239Pu) is formed.

As a result of successive absorption of neutrons in a nuclear reactor, elements even heavier than plutonium can be produced.

In natural minerals and uranium ore, only trace amounts of 239Pu, 244Pu and 237Np were found, so transuranium elements (heavier than uranium) are practically not found in the natural environment.

Uranium isotopes that exist in nature are not entirely stable with respect to b-decay and spontaneous fission, but they decay very slowly: half life uranium-238 is 4.5 billion years, and uranium-235 is 710 million years. Due to the low frequency of nuclear reactions, such long-lived isotopes are not dangerous sources of radiation. An ingot of natural uranium can be held in the hands without harm to health. Its specific activity is equal to 0.67 mCi/kg (Ci - curie, non-systemic unit of activity, equal to 3.7 * 1010 decays per second).

Uranium is a radioactive metal. In nature, uranium consists of three isotopes: uranium-238, uranium-235 and uranium-234. The highest level of stability is recorded for uranium-238.

radioactive decay of uranium

Radioactive decay is the process of a sudden change in the composition or internal structure of atomic nuclei, which are characterized by instability. In this case, elementary particles, gamma quanta and/or nuclear fragments are emitted. Radioactive substances contain a radioactive nucleus. The daughter nucleus resulting from radioactive decay can also become radioactive and, after a certain time, undergoes decay. This process continues until a stable nucleus devoid of radioactivity is formed. E. Rutherford experimentally proved in 1899 that uranium salts emit three types of rays:

• α-rays - a stream of positively charged particles
• β-rays - a stream of negatively charged particles
• γ-rays - do not create deviations in the magnetic field.

Radioactivity of uranium

Natural radioactivity is what distinguishes radioactive uranium from other elements. Uranium atoms, regardless of any factors and conditions, gradually change. In this case, invisible rays are emitted. After the transformations that occur with uranium atoms, a different radioactive element is obtained and the process is repeated. He will repeat as many times as necessary to get a non-radioactive element. For example, some chains of transformations have up to 14 stages. In this case, the intermediate element is radium, and the last stage is the formation of lead. This metal is not a radioactive element, so a number of transformations are interrupted. However, it takes several billion years for the complete transformation of uranium into lead.
Radioactive uranium ore often causes poisoning at enterprises involved in the extraction and processing of uranium raw materials. In the human body, uranium is a general cellular poison. It mainly affects the kidneys, but liver and gastrointestinal lesions also occur.
Uranium does not have completely stable isotopes. The longest lifetime is noted for uranium-238. The semi-decay of uranium-238 occurs over 4.4 billion years. A little less than one billion years is the half-decay of uranium-235 - 0.7 billion years. Uranium-238 occupies over 99% of the total volume of natural uranium. Due to its colossal half-life, the radioactivity of this metal is not high, for example, alpha particles cannot penetrate the stratum corneum of human skin. After a series of studies, scientists found that the main source of radiation is not uranium itself, but the radon gas formed by it, as well as its decay products that enter the human body during breathing.

Uranium-235(English uranium-235), historical name actinouranium(lat. Actin Uranium, indicated by the symbol AcU) is a radioactive nuclide of the chemical element uranium with atomic number 92 and mass number 235. The isotopic abundance of uranium-235 in nature is 0.7200 (51)%. It is the ancestor of the radioactive family 4n + 3, called the actinium series. Opened in 1935 by Arthur Dempster. Arthur Jeffrey Dempster.

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

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

Main article: Nuclear fission Yield curve of uranium-235 fission products for various energies of fissile neutrons.

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 with increasing excitation energy of the fissile nucleus, 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

Main article: 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. Under 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 neutron moderator substance to increase the concentration of thermal neutrons.

Isomers

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

• Excess mass: 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. 12345 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. 123456789101112 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. 123 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.

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Uranium is a radioactive metal. In nature, uranium consists of three isotopes: uranium-238, uranium-235 and uranium-234. The highest level of stability is recorded for uranium-238.

radioactive decay of uranium

Radioactive decay is the process of a sudden change in the composition or internal structure of atomic nuclei, which are characterized by instability. In this case, elementary particles, gamma quanta and/or nuclear fragments are emitted. Radioactive substances contain a radioactive nucleus. The daughter nucleus resulting from radioactive decay can also become radioactive and, after a certain time, undergoes decay. This process continues until a stable nucleus devoid of radioactivity is formed. E. Rutherford experimentally proved in 1899 that uranium salts emit three types of rays:

• α-rays - a stream of positively charged particles
• β-rays - a stream of negatively charged particles
• γ-rays - do not create deviations in the magnetic field.

Radioactivity of uranium

Natural radioactivity is what distinguishes radioactive uranium from other elements. Uranium atoms, regardless of any factors and conditions, gradually change.

Uranus (element)

In this case, invisible rays are emitted. After the transformations that occur with uranium atoms, a different radioactive element is obtained and the process is repeated. He will repeat as many times as necessary to get a non-radioactive element. For example, some chains of transformations have up to 14 stages. In this case, the intermediate element is radium, and the last stage is the formation of lead. This metal is not a radioactive element, so a number of transformations are interrupted. However, it takes several billion years for the complete transformation of uranium into lead.
Radioactive uranium ore often causes poisoning at enterprises involved in the extraction and processing of uranium raw materials. In the human body, uranium is a general cellular poison. It mainly affects the kidneys, but liver and gastrointestinal lesions also occur.
Uranium does not have completely stable isotopes. The longest lifetime is noted for uranium-238. The semi-decay of uranium-238 occurs over 4.4 billion years. A little less than one billion years is the half-decay of uranium-235 - 0.7 billion years. Uranium-238 occupies over 99% of the total volume of natural uranium. Due to its colossal half-life, the radioactivity of this metal is not high, for example, alpha particles cannot penetrate the stratum corneum of human skin. After a series of studies, scientists found that the main source of radiation is not uranium itself, but the radon gas formed by it, as well as its decay products that enter the human body during breathing.

radioactive uranium, radioactivity, radioactive decay

Isotopes and uranium production

Natural uranium consists of a mixture of three isotopes: 238U- 99.2739% (half-life T 1/2 \u003d 4.468 × 109 years), 235U - 0.7024% ( T 1/2 = 7.038×108 years) and 234U - 0.0057% ( T 1/2 = 2.455×105 years). The last isotope is not primary, but radiogenic; it is part of the 238U radioactive series.

The radioactivity of natural uranium is mainly due to the isotopes 238U and 234U; in equilibrium, their specific activities are equal. The specific activity of the isotope 235U in natural uranium is 21 times less than the activity of 238U.

There are 11 known artificial radioactive isotopes of uranium with mass numbers from 227 to 240. The longest-lived of them is 233U ( T 1/2 \u003d 1.62 × 105 years) is obtained by irradiating thorium neutrons and is capable of spontaneous fission by thermal neutrons.

The uranium isotopes 238U and 235U are the progenitors of two radioactive series. The final elements of these series are the lead isotopes 206Pb and 207Pb.

Under natural conditions, the isotopes 234U are mainly distributed: 235U: 238U = 0.0054: 0.711: 99.283. Half of the radioactivity of natural uranium is due to the 234U isotope. The 234U isotope is produced by the decay of 238U. For the last two, unlike other pairs of isotopes and regardless of the high migration ability of uranium, the geographical constancy of the ratio U238 / U235 = 137.88 is characteristic. The value of this ratio depends on the age of uranium. Numerous natural measurements showed its insignificant fluctuations. So in rolls, the value of this ratio relative to the standard varies within 0.9959 - 1.0042, in salts - 0.996 - 1.005. In uranium-containing minerals (nasturan, uranium black, cirtholite, rare-earth ores), the value of this ratio varies between 137.30 - 138.51; moreover, the difference between the forms UIV and UVI has not been established; in sphene - 138.4. In some meteorites, a lack of the 235U isotope was revealed. Its lowest concentration under terrestrial conditions was found in 1972 by the French researcher Buzhiges in the Oklo town in Africa (deposit in Gabon). Thus, normal uranium contains 0.7025% uranium 235U, while in Oklo it decreases to 0.557%. This confirmed the hypothesis of a natural nuclear reactor leading to isotope burnup, predicted by George W. Wetherill of the University of California at Los Angeles and Mark G. Inghram of the University of Chicago and Paul K. Kuroda , a chemist at the University of Arkansas, who described the process back in 1956. In addition, natural nuclear reactors have been found in the same districts: Okelobondo, Bangombe, and others. Currently, about 17 natural nuclear reactors are known.

Receipt

The very first stage of uranium production is concentration. The rock is crushed and mixed with water. Heavy suspended matter components settle out faster. If the rock contains primary uranium minerals, they precipitate quickly: these are heavy minerals. Secondary uranium minerals are lighter, in which case heavy waste rock settles earlier. (However, it is far from always really empty; it can contain many useful elements, including uranium).

The next stage is the leaching of concentrates, the transfer of uranium into solution. Apply acid and alkaline leaching. The first is cheaper, since sulfuric acid is used to extract uranium. But if in the feedstock, as, for example, in uranium tar, uranium is in a tetravalent state, then this method is not applicable: tetravalent uranium in sulfuric acid practically does not dissolve. In this case, one must either resort to alkaline leaching, or pre-oxidize uranium to the hexavalent state.

Do not use acid leaching and in cases where the uranium concentrate contains dolomite or magnesite, reacting with sulfuric acid.

In these cases, caustic soda (hydroxydosodium) is used.

The problem of uranium leaching from ores is solved by oxygen purge. An oxygen flow is fed into a mixture of uranium ore with sulfide minerals heated to 150 °C. In this case, sulfuric acid is formed from sulfur minerals, which washes out uranium.

At the next stage, uranium must be selectively isolated from the resulting solution. Modern methods - extraction and ion exchange - allow to solve this problem.

The solution contains not only uranium, but also other cations. Some of them under certain conditions behave in the same way as uranium: they are extracted with the same organic solvents, deposited on the same ion-exchange resins, and precipitate under the same conditions. Therefore, for the selective isolation of uranium, one has to use many redox reactions in order to get rid of one or another undesirable companion at each stage. On modern ion-exchange resins, uranium is released very selectively.

Methods ion exchange and extraction they are also good because they allow you to fully extract uranium from poor solutions (the uranium content is tenths of a gram per liter).

After these operations, uranium is transferred to a solid state - into one of the oxides or into UF4 tetrafluoride. But this uranium still needs to be purified from impurities with a large thermal neutron capture cross section - boron, cadmium, hafnium. Their content in the final product should not exceed hundred thousandths and millionths of a percent. To remove these impurities, a commercially pure uranium compound is dissolved in nitric acid. In this case, uranyl nitrate UO2(NO3)2 is formed, which, upon extraction with tributyl phosphate and some other substances, is additionally purified to the required conditions. Then this substance is crystallized (or peroxide UO4 2H2O is precipitated) and carefully ignited. As a result of this operation, uranium trioxide UO3 is formed, which is reduced with hydrogen to UO2.

Uranium dioxide UO2 at a temperature of 430 to 600 °C is treated with dry hydrogen fluoride to obtain UF4 tetrafluoride. Metallic uranium is reduced from this compound using calcium or magnesium.

depleted uranium

After extracting 235U and 234U from natural uranium, the remaining material (uranium-238) is called "depleted uranium" because it is depleted in the 235 isotope. According to some reports, about 560,000 tons of depleted uranium hexafluoride (UF6) are stored in the United States.

Depleted uranium is half as radioactive as natural uranium, mainly due to the removal of 234U from it. Because the main use of uranium is energy production, depleted uranium is a low-use product with low economic value.

Basically, its use is associated with the high density of uranium and its relatively low cost. Depleted uranium is used for radiation shielding (ironically) and as ballast in aerospace applications such as aircraft control surfaces. Each Boeing 747 aircraft contains 1,500 kg of depleted uranium for this purpose. This material is also used in high-speed gyroscope rotors, large flywheels, as ballast in space descent vehicles and racing yachts, while drilling oil wells.

Physiological action

In microquantities (10−5-10−8%) found in the tissues of plants, animals and humans. It accumulates to the greatest extent by some fungi and algae. Uranium compounds are absorbed in the gastrointestinal tract (about 1%), in the lungs - 50%. The main depots in the body: the spleen, kidneys, skeleton, liver, lungs and broncho-pulmonary lymph nodes. The content in organs and tissues of humans and animals does not exceed 10−7g.

Uranium and its compounds are toxic. Aerosols of uranium and its compounds are especially dangerous. For aerosols of water-soluble uranium compounds MPC in air is 0.015 mg/m³, for insoluble forms of uranium MPC is 0.075 mg/m³. When it enters the body, uranium acts on all organs, being a general cellular poison. Uranium almost irreversibly, like many other heavy metals, binds to proteins, primarily to the sulfide groups of amino acids, disrupting their function. The molecular mechanism of action of uranium is associated with its ability to inhibit the activity of enzymes. First of all, the kidneys are affected (protein and sugar appear in the urine, oliguria). With chronic intoxication, hematopoietic and nervous system disorders are possible.

Uranium mining in the world

10 countries responsible for 94% of the world's uranium production

According to the "Red Book of Uranium" issued by the OECD, 41,250 tons of uranium were mined in 2005 (in 2003 - 35,492 tons). According to the OECD, there are 440 commercial reactors operating in the world, which consume 67,000 tons of uranium per year. This means that its production provides only 60% of its consumption (the rest is recovered from old nuclear warheads). Production by countries in tons by U content for 2005-2006 (see table No. 13, Appendix A).

Production in Russia

In the USSR, the main uranium ore regions were Ukraine (the Zheltorechenskoye, Pervomayskoye deposits, etc.), Kazakhstan (Northern - Balkashinskoe ore field, etc.; Southern - Kyzylsay ore field, etc.; Vostochny; all of them belong mainly to the volcanogenic-hydrothermal type); Transbaikalia (Antey, Streltsovskoye, etc.); Central Asia, mainly Uzbekistan with mineralization in black shales with a center in the city of Uchkuduk. There are many small ore occurrences and manifestations. In Russia, Transbaikalia remained the main uranium-ore region. About 93% of Russian uranium is mined at the deposit in the Chita region (near the city of Krasnokamensk). Mining is carried out by the Priargunsky Industrial Mining and Chemical Association (PIMCU), which is part of JSC Atomredmetzoloto (Uranium Holding), using the mine method.

The remaining 7% is obtained by in-situ leaching from ZAO Dalur (Kurgan Region) and OAO Khiagda (Buryatia).

The resulting ores and uranium concentrate are processed at the Chepetsk Mechanical Plant.

Mining in Kazakhstan

About a fifth of the world's uranium reserves are concentrated in Kazakhstan (21% and 2nd place in the world). The total resources of uranium are about 1.5 million tons, of which about 1.1 million tons can be mined by in-situ leaching.

In 2009, Kazakhstan came out on top in the world in terms of uranium mining (13,500 tons were mined).

Production in Ukraine

The main enterprise is the Eastern Mining and Processing Plant in the city of Zhovti Vody.

Application

Although uranium-238 cannot be used as a primary fissile material, due to the high energy of the neutrons required for its fission, it has an important place in the nuclear industry.

Having a high density and atomic weight, U-238 is suitable for making reflector charge shells from it in fusion and fission devices. The fact that it is fissile with fast neutrons increases the energy yield of the charge: indirectly, by multiplying reflected neutrons; directly during the fission of shell nuclei by fast neutrons (during synthesis). Approximately 40% of the neutrons produced during fission and all fusion neutrons have sufficient energies for U-238 fission.

U-238 has a spontaneous fission rate 35 times higher than U-235, 5.51 fiss/s*kg. This makes it impossible to use it as a shell of a reflector charge in cannon bombs, because its suitable mass (200-300 kg) will create too high a neutron background.

Pure U-238 has a specific radioactivity of 0.333 microcurie/g.

An important field of application of this uranium isotope is the production of plutonium-239. Plutonium is formed in the course of several reactions that begin after the capture of a neutron by a U-238 atom. Any reactor fuel containing natural or partially enriched uranium in the 235th isotope contains a certain proportion of plutonium after the end of the fuel cycle.

Decay chain of uranium-238

The isotope uranium-238, it is more than 99% in natural uranium. This isotope is also the most stable; thermal neutrons cannot split its nucleus. In order to split 238U, the neutron needs an additional kinetic energy of 1.4 MeV. A nuclear reactor made of pure uranium-238 will not work under any circumstances.

An atom of uranium-238, in whose nucleus the protons and neutrons are barely held together by cohesive forces. From time to time, a compact group of four particles escapes from it: two protons and two neutrons (α-particle). Uranium-238 is thus converted into thorium-234, which contains 90 protons and 144 neutrons in its nucleus. But thorium-234 is also unstable. Its transformation, however, is not the same as in the previous case: one of its neutrons turns into a proton, and thorium-234 turns into protactinium-234, the nucleus of which contains 91 protons and 143 neutrons. This metamorphosis, which took place in the nucleus, also affects the electrons moving in their orbits: one of them becomes unpaired and flies out of the atom. Protactinium is very unstable and takes very little time to transform. This is followed by other transformations, accompanied by radiation, and this whole chain, in the end, ends with a stable lead nuclide (see Figure No. 7, Appendix B).

The most important circumstance for nuclear energy is that the most common uranium isotope, 238U, is also a potential source of nuclear fuel. Both Szilard and Fermi were right in assuming that the absorption of neutrons by uranium would lead to the formation of new elements.

Isotopes of uranium

Indeed, in a collision with a thermal neutron, uranium-238 does not fission; instead, the nucleus absorbs the neutron. On average, in 23.5 minutes, one of the neutrons in the nucleus turns into a proton (with the emission of an electron, the β-decay reaction), and the nucleus of uranium-239 becomes the nucleus of neptunium-239 (239Np). After 2.4 days, the second β-decay occurs and plutonium-239 (239Pu) is formed.

As a result of successive absorption of neutrons in a nuclear reactor, elements even heavier than plutonium can be produced.

In natural minerals and uranium ore, only trace amounts of 239Pu, 244Pu and 237Np were found, so transuranium elements (heavier than uranium) are practically not found in the natural environment.

Uranium isotopes that exist in nature are not entirely stable with respect to alpha decay and spontaneous fission, but they decay very slowly: half life uranium-238 is 4.5 billion years, and uranium-235 is 710 million years. Due to the low frequency of nuclear reactions, such long-lived isotopes are not dangerous sources of radiation. An ingot of natural uranium can be held in the hands without harm to health. Its specific activity is equal to 0.67 mCi/kg (Ci - curie, non-systemic unit of activity, equal to 3.7 * 1010 decays per second).

Receipt - uranium

Page 1

Obtaining uranium from the ashes of domestic coal - the newspaper wrote - can be considered a resolved issue. 1 ton of ash from some coals contains atomic energy equivalent to 6,000 tons of coal.

Obtaining uranium, gold; separation of uranium fission products; obtaining non-ferrous metals and rare earth elements.

Obtaining uranium and thorium is preceded by a complex complex processing of ore raw materials.

To obtain uranium, solid UF4 is reduced with calcium or magnesium.

It is used to obtain uranium, thorium and other metals, as well as in organic synthesis.

The energy consumption for obtaining uranium by ideal quenching of the reaction mixture is 71 eV per metal atom.

The main source of uranium is the mineral uraninite and its varieties - resin blende, uranium mica, pitchblende, uranium black. Of great importance for the production of uranium and its compounds are uranium-vanadium, uranium-phosphorus, uranium-arsenic-acid salts of calcium, copper, barium, called uranium mica.

In recent years, underground leaching with subsequent purification of solutions has been used to obtain uranium. Sulfuric acid and carbonate solutions are used for underground leaching.

Shale deposited in Tennessee, Kentucky, Indiana, Illinois, and Ohio are another major potential source of uranium in the United States.

There are many other ways to obtain uranium tetrafluoride, including the reaction of interaction of hydrogen fluoride with compact metallic uranium in a hydrogen atmosphere, starting at 250 C.

There are practically no methods for calculating crucible furnaces for producing uranium. When designing them, one can only take into account such factors as the amount of heat released by the reaction and lost to the surrounding space, as well as (in the case of magnesium reduction) the amount of heat that must be supplied using external heaters.

Japan has developed a new technology for producing uranium from a solution of phosphoric acid used to produce phosphate fertilizers. Prior to the construction of a plant for the extraction of uranium from 3-4 million tons of phosphates imported annually by Japan as a raw material for the production of fertilizers, it is planned to build a pilot plant.

It should be emphasized that the process of obtaining uranium is not as simple as it is described here. It should be remembered that all processes are carried out in complex equipment made of special materials. In this case, a very precise dosage of reagents must be observed and the required temperature must be maintained. The uranium production process requires a large quantity of extremely pure reagents, which must be purer than the so-called chemically pure substances.

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