Nuclear technologies are largely based on the use of radiochemistry methods, which in turn are based on the nuclear-physical, physical, chemical and toxic properties of radioactive elements.

In this chapter, we will limit ourselves brief description properties of the main fissile isotopes - uranium and plutonium.

Uranus

Uranus ( uranium) U - actinide group element, 7-0th period periodic system, Z=92, atomic mass 238.029; the heaviest of those found in nature.

There are 25 known isotopes of uranium, all of which are radioactive. The easiest 217U (Tj/ 2 = 26 ms), the heaviest 2 4 2 U (7 T J / 2 = i6.8 min). There are 6 nuclear isomers. There are three radioactive isotopes in natural uranium: 2 s 8 and (99.2 739%, Ti/ 2 = 4.47109 l), 2 35U (0.7205%, G, / 2 = 7.04-109 years) and 2 34U ( 0.0056%, Ti/ 2=2.48-swl). The specific radioactivity of natural uranium is 2.48104 Bq, divided almost in half between 2 34U and 288 U; 235U makes a small contribution (the specific activity of the isotope 233 in natural uranium is 21 times less than the activity of 238U). The thermal neutron capture cross section is 46, 98, and 2.7 barn for 2 zz, 2 35U, and 2 3 8 U, respectively; fission cross section 527 and 584 barn for 2 zz and 2 s 8 and, respectively; natural mixture of isotopes (0.7% 235U) 4.2 barn.

Tab. 1. Nuclear physical properties 2 h9 Ri and 2 35C.

Tab. 2. Neutron capture 2 35C and 2 h 8 C.

Six isotopes of uranium are capable of spontaneous fission: 282 U, 2 szy, 234U, 235U, 2 s 6 u and 2 s 8 u. The natural isotopes 233 and 235U fission under the action of both thermal and fast neutrons, while nuclei 238 and are capable of fission only when neutrons with an energy of more than 1.1 MeV are captured. When neutrons with a lower energy are captured, the 288 U nuclei are first converted into 2 -i9U nuclei, which then undergo p-decay and go first into 2 -" * 9Np, and then into 2 39Pu. Effective cross sections for the capture of thermal neutrons of 2 34U, 2 35U and 2 3 8 and are equal to 98, 683 and 2.7-barns, respectively. Complete fission of 2 35U leads to a "thermal energy equivalent" of 2-107 kWh / kg. The isotopes 2 35U and 2 zzy are used as nuclear fuel, capable of supporting fission chain reaction.

Nuclear reactors produce n artificial isotopes of uranium with mass numbers 227-240, of which the longest-lived is 233U (7 V 2 \u003d i.62 *io 5 years); it is obtained by neutron irradiation of thorium. Uranium isotopes with mass numbers 239^257 are born in the superpowerful neutron fluxes of a thermonuclear explosion.

Uranium-232- technogenic nuclide, a-emitter, T x / 2=68.9 years, parent isotopes 2 3 6 Pu(a), 23 2 Np(p*) and 23 2 Pa(p), daughter nuclide 228 Th. The intensity of spontaneous fission is 0.47 divisions / s kg.

Uranium-232 is formed as a result of the following decays:

P + - decay of the nuclide * 3 a Np (Ti / 2 \u003d 14.7 min):

In the nuclear industry, 2 3 2 U is produced as a by-product in the synthesis of the fissile (weapon-grade) nuclide 2 33 in the thorium fuel cycle. When irradiated with 2 3 2 Th neutrons, the main reaction occurs:

and side two-step reaction:

The production of 232 U from thorium occurs only on fast neutrons (E„>6 MeV). If there is 2 s°Th in the initial substance, then the formation of 2 3 2 U is supplemented by the reaction: 2 s°Th + u-> 2 3'Th. This reaction takes place on thermal neutrons. Generation 2 3 2 U is undesirable for a number of reasons. It is suppressed by the use of thorium with a minimum concentration of 23°Th.

The decay of 2 from 2 occurs in the following directions:

A decay in 228 Th (probability 100%, decay energy 5.414 MeV):

the energy of emitted a-particles is 5.263 MeV (in 31.6% of cases) and 5.320 MeV (in 68.2% of cases).

• - spontaneous fission (probability less than ~ 12%);
• - cluster decay with the formation of the nuclide 28 Mg (the probability of decay is less than 5 * 10 "12%):

Cluster decay with the formation of nuclide 2

Uranium-232 is the ancestor of a long decay chain, which includes nuclides - emitters of hard y-quanta:

^U-(3.64 days, a, y)-> 220 Rn-> (55.6 s, a)-> 21b Po->(0.155 s, a)-> 212 Pb->(10.64 h , p, y) -> 212 Bi -> (60.6 m, p, y) -> 212 Po a, y) -> 208x1, 212 Po -> (3" 10' 7 s, a) -> 2o8 Pb (stub), 2o8 T1 -> (3.06 m, p, y -> 2o8 Pb.

The accumulation of 2 3 2 U is inevitable in the production of 2 zzy in the thorium energy cycle. Intense y-radiation arising from the decay of 2 3 2 U hinders the development of thorium energy. It is unusual that the even isotope 2 3 2 11 has a high fission cross section under the action of neutrons (75 barn for thermal neutrons), as well as a high neutron capture cross section - 73 barn. 2 3 2 U is used in the method of radioactive tracers in chemical research.

2 z 2 and is the ancestor of a long decay chain (according to the scheme 2 z 2 Th), which includes nuclides emitting hard y-quanta. The accumulation of 2 3 2 U is inevitable in the production of 2 zzy in the thorium energy cycle. Intense γ-radiation arising from the decay of 232 U hinders the development of thorium energy. Unusual is that the even isotope 2 3 2 U has a high fission cross section under the action of neutrons (75 barn for thermal neutrons), as well as a high neutron capture cross section - 73 barn. 2 3 2 U is often used in the method of radioactive tracers in chemical and physical research.

Uranium-233- technogenic radionuclide, a-emitter (energies 4.824 (82.7%) and 4.783 MeV (14.9%),), Tvi= 1.585105 years, parent nuclides 2 37Pu(a)-? 2 33Np(p +) -> 2 33Pa(p), daughter nuclide 22 9Th. 2 zzi is obtained in nuclear reactors from thorium: 2 s 2 Th captures a neutron and turns into 2 zz Th, which decays into 2 zz Pa, and then into 2 zz. Nuclei 2 zzy (odd isotope) are capable of both spontaneous fission and fission under the action of neutrons of any energy, which makes it suitable for the production of both atomic weapons and reactor fuel. The effective fission cross section is 533 barn, the capture cross section is 52 barn, the neutron yield is 2.54 per fission event, and 2.31 per absorbed neutron. The critical mass of 2 zz is three times less than the critical mass of 2 35U (-16 kg). The intensity of spontaneous fission is 720 cases / s kg.

Uranium-233 is formed as a result of the following decays:

- (3 + -decay of nuclide 2 33Np (7^=36.2 min):

On an industrial scale, 2 zzi is obtained from 2 32Th by neutron irradiation:

When a neutron is absorbed, the 234 nucleus usually fissions, but occasionally captures a neutron, turning into 234U. Although 2 zzy, having absorbed a neutron, usually fissions, nevertheless, it sometimes retains a neutron, turning into 2 34U. The operating time of 2 zz is carried out both in fast and in thermal reactors.

From a weapon point of view, 2 zzi is comparable to 2 39 Pu: its radioactivity is 1/7 of the activity of 2 39 Pu (Ti/ 2 \u003d 159200 l versus 24100 l for Pu), the critical mass of 2 szi is 6o% higher than that of IgPu (16 kg versus 10 kg), and the rate of spontaneous fission is 20 times higher (b-u - ' versus 310 10). The neutron flux from 239Pu is 3 times higher than that from 239Pu. The creation of a nuclear charge on the basis of 2 sz requires more effort than on ^Pu. The main obstacle is the presence of the 232U impurity in 2zzi, the y-radiation of the decay projects of which makes it difficult to work with 2zzi and makes it easy to detect ready-made weapons. In addition, the short half-life of 2 3 2 U makes it an active source of a-particles. 2 zzi with 1% 232 and has 3 times stronger a-activity than weapons-grade plutonium and, accordingly, greater radiotoxicity. This a-activity causes the birth of neutrons in the light elements of the weapon charge. To minimize this problem, the presence of such elements as Be, B, F, Li should be minimal. The presence of a neutron background does not affect the operation of implosion systems, but for gun schemes a high level of purity for light elements is required. zgi is not harmful, and even desirable, because it reduces the possibility of using uranium for weapons purposes.After processing the spent nuclear fuel and reusing the fuel, the content of 232U reaches 0.1 + 0.2%.

The decay of 2 zzy occurs in the following directions:

A-decay in 22 9Th (probability 100%, decay energy 4.909 MeV):

the energy of the emitted n-particles is 4.729 MeV (in 1.61% of cases), 4.784 MeV (in 13.2% of cases) and 4.824 MeV (in 84.4% of cases).

• - spontaneous fission (probability
• - cluster decay with the formation of the nuclide 28 Mg (the probability of decay is less than 1.3*10 -13%):

Cluster decay with the formation of the nuclide 24 Ne (decay probability 7.3-10-“%):

The 2 zz decay chain belongs to the Neptunium series.

The specific radioactivity is 2 zzi 3.57-8 Bq/g, which corresponds to an a-activity (and radiotoxicity) of -15% of plutonium. Only 1% 2 3 2 U increases the radioactivity to 212 mCi/g.

Uranium-234(Uranus II, UII) is a part of natural uranium (0.0055%), 2.445105 years, a-emitter (energy of a-particles 4.777 (72%) and

4.723 (28%) MeV), parent radionuclides: 2 s 8 Pu(a), 234 Pa(P), 234 Np(p +),

daughter isotope in 2 s"t.

Usually 234 U is in equilibrium with 2 3 8 u, decaying and forming at the same rate. Approximately half of the radioactivity of natural uranium is the contribution of 234U. Usually 234U is obtained by ion-exchange chromatography of old preparations of pure 238 Pu. In a-decay, *34U yields to 234U, so the old preparations of 238Pu are good sources of 234U. 100 g 2s8Pu contain 776 mg 234U after a year, after 3 years

2.2 g 2 34U. The concentration of 2 34U in highly enriched uranium is quite high due to the preferential enrichment in light isotopes. Since 234u is a strong y-emitter, there are restrictions on its concentration in uranium intended for processing into fuel. Enhanced level 234 and is acceptable for reactors, but reprocessed SNF contains already unacceptable levels of this isotope.

The decay of 234u occurs along the following lines:

A-decay in 23°T (probability 100%, decay energy 4.857 MeV):

the energy of emitted a-particles is 4.722 MeV (in 28.4% of cases) and 4.775 MeV (in 71.4% of cases).

• - spontaneous fission (probability 1.73-10-9%).
• - cluster decay with the formation of the nuclide 28 Mg (the probability of decay is 1.4-10 "n%, according to other sources 3.9-10-"%):

• - cluster decay with the formation of nuclides 2 4Ne and 26 Ne (the probability of decay is 9-10 ", 2%, according to other data 2.3-10 - 11%):

The only isomer 2 34ti is known (Tx/ 2 = 33.5 μs).

The absorption cross section of 2 34U thermal neutrons is 10 barn, and for the resonance integral averaged over various intermediate neutrons, 700 barn. Therefore, in thermal neutron reactors, it is converted to fissile 235U at a faster rate than much more 238U (with a cross section of 2.7 barn) is converted into 2 s9Pu. As a result, SNF contains less 234U than fresh fuel.

Uranium-235 belongs to the 4P + 3 family, is capable of producing a fission chain reaction. This is the first isotope on which the reaction of forced fission of nuclei under the action of neutrons was discovered. Absorbing a neutron, 235U goes into 2 zbi, which is divided into two parts, releasing energy and emitting several neutrons. Fissile by neutrons of any energy, capable of spontaneous fission, the isotope 2 35U is part of natural uthanum (0.72%), a-emitter (energies 4.397 (57%) and 4.367 (18%) MeV), Ti/j=7.038-th 8 years, parent nuclides 2 35Pa, 2 35Np and 2 39Pu, daughter - 23"Th. The intensity of spontaneous fission 2 3su 0.16 divisions/s kg. The fission of one 2 35U nucleus releases 200 MeV of energy = 3.2 Yu p J, i.e. 18 TJ/mol=77 TJ/kg. The cross section of fission by thermal neutrons is 545 barns, and by fast neutrons - 1.22 barns, neutron yield: per fission event - 2.5, per absorbed neutron - 2.08.

Comment. The capture cross section of slow neutrons to form the isotope 2 si (10 barn), so that the total absorption cross section of slow neutrons is 645 barn.

• - spontaneous fission (probability 7*10~9%);
• - cluster decay with the formation of nuclides 2 °Ne, 2 5Ne and 28 Mg (the probabilities are respectively 8-io - 10%, 8-kg 10%, 8 * 10 ".0%):

Rice. 1.

The only isomer known is 2 35n»u (7/ 2 = 26 min).

Specific activity 2 35C 7.77-u 4 Bq/g. The critical mass of weapons-grade uranium (93.5% 2 35U) for a ball with a reflector is 15-7-23 kg.

Fission 2 » 5U is used in atomic weapons, for energy production, and for the synthesis of important actinides. The chain reaction is maintained due to the excess of neutrons produced during the fission of 2 35C.

Uranium-236 occurs on Earth in nature in trace amounts (on the Moon it is more), a-emitter (?

Rice. 2. Radioactive family 4/7+2 (including -3 8 and).

In an atomic reactor, 233 absorbs a thermal neutron, after which it fissions with a probability of 82%, and emits a y-quantum with a probability of 18% and turns into 236 and . In small quantities it is part of fresh fuel; accumulates when uranium is irradiated with neutrons in the reactor, and therefore is used as a SNF “signaling device”. 2 h b and is formed as a by-product during the separation of isotopes by gaseous diffusion during the regeneration of used nuclear fuel. The 236 U produced in the power reactor is a neutron poison, its presence in nuclear fuel is compensated high level enrichment 2 35U.

2b and is used as a mixing tracer for oceanic waters.

Uranium-237,T&= 6.75 days, beta and gamma emitter, can be obtained by nuclear reactions:

Detection 287 and carried out along lines with eu= o.v MeV (36%), 0.114 MeV (0.06%), 0.165 MeV (2.0%), 0.208 MeV (23%)

237U is used in the method of radioactive tracers in chemical research. Measurement of the concentration (2 4°Am) in the fallout from an atomic weapon test provides valuable information about the type of charge and the equipment used.

Uranium-238- belongs to the 4P + 2 family, fissile with high-energy neutrons (more than 1.1 MeV), capable of spontaneous fission, forms the basis of natural uranium (99.27%), a-emitter, 7'; /2=4>468-109 years, directly decomposes into 2 34Th, forms a number of genetically related radionuclides, and after 18 products turns into 206 Pb. Pure 2 3 8 U has a specific radioactivity of 1.22-104 Bq. The half-life is very long - about 10 16 years, so that the probability of fission in relation to the main process - the emission of an a-particle - is only 10 "7. One kilogram of uranium gives only 10 spontaneous fissions per second, and during the same time an a-particle emit 20 million nuclei Parent nuclides: 2 4 2 Pu(a), *spa(p-) 234Th, daughter T,/ 2 = 2 :i 4 th.

Uranium-238 is formed as a result of the following decays:

2 (V0 4) 2] 8Н 2 0. Of the secondary minerals, hydrated calcium uranyl phosphate Ca (U0 2) 2 (P0 4) 2 -8H 2 0 is common. Often uranium in minerals is accompanied by other useful elements - titanium, tantalum, rare earths. Therefore, it is natural to strive for the complex processing of uranium-containing ores.

Basic physical properties of uranium: atomic mass 238.0289 a.m.u. (g/mol); atomic radius 138 pm (1 pm = 12 m); ionization energy (first electron 7.11 eV; electronic configuration -5f36d‘7s 2; oxidation states 6, 5, 4, 3; G P l \u003d 113 2, 2 °; T t,1=3818°; density 19.05; specific heat capacity 0.115 JDKmol); tensile strength 450 MPa, heat of fusion 12.6 kJ/mol, heat of vaporization 417 kJ/mol, specific heat capacity 0.115 J/(mol-K); molar volume 12.5 cm3/mol; the characteristic Debye temperature © D = 200K, the transition temperature to the superconducting state is 0.68K.

Uranium is a heavy, silvery-white, glossy metal. It is slightly softer than steel, malleable, flexible, has slight paramagnetic properties, and is pyrophoric in the powdered state. Uranium has three allotropic forms: alpha (rhombic, a-U, lattice parameters 0=285, b= 587, c=49b pm, stable up to 667.7°), beta (tetragonal, p-U, stable from 667.7 to 774.8°), gamma (with a cubic body-centered lattice, y-U, existing from 774.8° to melting points, frm=ii34 0), at which uranium is most malleable and convenient for processing.

At room temperature, the rhombic a-phase is stable, the prismatic structure consists of wavy atomic layers parallel to the plane abc, in an extremely asymmetric prismatic lattice. Within the layers, the atoms are closely bonded, while the strength of the bonds between the atoms of adjacent layers is much weaker (Fig. 4). This anisotropic structure makes it difficult to fuse uranium with other metals. Only molybdenum and niobium create solid-state alloys with uranium. Yet metallic uranium can interact with many alloys, forming intermetallic compounds.

In the interval 668 ^ 775 ° there is a (3-uranium. Tetragonal type lattice has a layered structure with layers parallel to the plane ab in positions 1/4С, 1/2 With and 3/4C unit cell. At temperatures above 775°, y-uranium is formed with a body-centered cubic lattice. The addition of molybdenum makes it possible to have the y-phase at room temperature. Molybdenum forms a wide range of solid solutions with y-uranium and stabilizes the y-phase at room temperature. y-Uranium is much softer and more malleable than the brittle a- and (3-phases.

Neutron irradiation has a significant effect on the physical and mechanical properties of uranium, causing an increase in the size of the sample, a change in shape, as well as a sharp deterioration in the mechanical properties (creep, embrittlement) of uranium blocks during the operation of a nuclear reactor. The increase in volume is due to the accumulation in uranium during fission of impurities of elements with a lower density (translation 1% uranium into fragmentation elements increases the volume by 3.4%).

Rice. 4. Some crystal structures of uranium: a - a-uranium, b - p-uranium.

The most common methods for obtaining uranium in the metallic state are the reduction of their fluorides with alkali or alkaline earth metals or the electrolysis of their salt melts. Uranium can also be obtained by metallothermic reduction from carbides with tungsten or tantalum.

The ability to easily donate electrons determines the reducing properties of uranium and its high chemical activity. Uranium can interact with almost all elements, except noble gases, while acquiring oxidation states +2, +3, +4, +5, +6. In solution, the main valency is 6+.

Rapidly oxidizing in air, metallic uranium is covered with an iridescent film of oxide. Fine powder of uranium ignites spontaneously in air (at temperatures of 1504-175°), forming and;) Ov. At 1000°, uranium combines with nitrogen to form yellow uranium nitride. Water is able to react with metal slowly at low temperatures and rapidly at high temperatures. Uranium reacts violently with boiling water and steam to release hydrogen, which forms a hydride with uranium.

This reaction is more vigorous than the combustion of uranium in oxygen. Such chemical activity of uranium makes it necessary to protect uranium in nuclear reactors from contact with water.

Uranium dissolves in hydrochloric, nitric and other acids, forming U(IV) salts, but does not interact with alkalis. Uranium displaces hydrogen from inorganic acids and salt solutions of metals such as mercury, silver, copper, tin, platinum and gold. With strong shaking, the metal particles of uranium begin to glow.

Features of the structure of the electron shells of the uranium atom (the presence of ^/-electrons) and some of its physico-chemical properties serve as the basis for classifying uranium as an actinide. However, there is a chemical analogy between uranium and Cr, Mo, and W. Uranium is highly reactive and reacts with all elements except the noble gases. In the solid phase, examples of U(VI) are uranyl trioxide U0 3 and uranyl chloride U0 2 C1 2 . Uranium tetrachloride UC1 4 and uranium dioxide U0 2

U(IV) examples. Substances containing U(IV) are usually unstable and become hexavalent upon prolonged exposure to air.

Six oxides are installed in the uranium-oxygen system: UO, U0 2 , U 4 0 9 , and 3 Ov, U0 3 . They are characterized by a wide area of ​​homogeneity. U0 2 is a basic oxide, while U0 3 is amphoteric. U0 3 - interacts with water to form a number of hydrates, the most important of which are diuronic acid H 2 U 2 0 7 and uranic acid H 2 1U 4. With alkalis, U0 3 forms salts of these acids - uranates. When U0 3 is dissolved in acids, salts of the doubly charged uranyl cation U0 2 a+ are formed.

Uranium dioxide, U0 2 , is brown in stoichiometric composition. As the oxygen content in the oxide increases, the color changes from dark brown to black. Crystal structure of CaF 2 type, A = 0.547 nm; density 10.96 g / cm "* (the highest density among uranium oxides). T , pl \u003d 2875 0, T kn „ \u003d 3450 °, D # ° 298 \u003d -1084.5 kJ / mol. Uranium dioxide is a semiconductor with hole conductivity, a strong paramagnet. MAC = 0.015 mg/m3. Let's not dissolve in water. At a temperature of -200° it adds oxygen, reaching the composition U0 2>25.

Uranium (IV) oxide can be obtained by reactions:

Uranium dioxide exhibits only basic properties, it corresponds to the basic hydroxide U (OH) 4, which then turns into hydrated hydroxide U0 2 H 2 0. Uranium dioxide slowly dissolves in strong non-oxidizing acids in the absence of atmospheric oxygen to form W + ions:

U0 2 + 2H 2 S0 4 ->U(S0 4) 2 + 2Н 2 0. (38)

It is soluble in concentrated acids, and the dissolution rate can be greatly increased by the addition of fluorine ion.

When dissolved in nitric acid, the uranyl ion 1U 2 2+ is formed:

Triuran octoxide U 3 0s (uranium oxide) - powder, the color of which varies from black to dark green; at strong crushing - olive-green color. Large black crystals leave green strokes on porcelain. There are three known crystalline modifications of U 3 0 h: a-U 3 C>8 - rhombic crystal structure (sp. gr. C222; 0=0.671 nm; 6=1.197 nm; c=0.83 nm; d =0.839 nm); p-U 3 0e - rhombic crystal structure (space group Stst; 0=0.705 nm; 6=1.172 nm; 0=0.829 nm. The beginning of decomposition is 100° (goes to 110 2), MPC = 0.075 mg / m3.

U 3 C>8 can be obtained by the reaction:

By calcining U0 2, U0 2 (N0 3) 2, U0 2 C 2 0 4 3H 2 0, U0 4 -2H 2 0 or (NH 4) 2 U 2 0 7 at 750 0 in air or in an oxygen atmosphere (p = 150 + 750 mm Hg) receive stoichiometrically pure U 3 08.

When U 3 0s is calcined at T > 100°, it is reduced to 110 2, however, when cooled in air, it returns to U 3 0s. U 3 0e dissolves only in concentrated strong acids. In hydrochloric and sulfuric acids, a mixture of U(IV) and U(VI) is formed, and in nitric acid, uranyl nitrate is formed. diluted sulfuric and hydrochloric acid react very weakly with U 3 Os even when heated, the addition of oxidizing agents (nitric acid, pyrolusite) sharply increases the dissolution rate. Concentrated H 2 S0 4 dissolves U 3 Os with the formation of U(S0 4) 2 and U0 2 S0 4 . Nitric acid dissolves U 3 Oe with the formation of uranyl nitrate.

Uranium trioxide, U0 3 - crystalline or amorphous substance of bright yellow color. Reacts with water. MPC \u003d 0.075 mg / m 3.

It is obtained by calcining ammonium polyuranates, uranium peroxide, uranyl oxalate at 300-500 ° and hexahydrate uranyl nitrate. In this case, an orange powder of an amorphous structure is formed with a density

6.8 g/cm. The crystalline form IO 3 can be obtained by the oxidation of U 3 0 8 at temperatures of 450°-750° in an oxygen stream. There are six crystalline modifications of U0 3 (a, (3, y> §> ?, n) - U0 3 is hygroscopic and turns into uranyl hydroxide in moist air. further heating to 6oo° makes it possible to obtain U 3 Os.

Hydrogen, ammonia, carbon, alkali and alkaline earth metals reduce U0 3 to U0 2 . By passing a mixture of HF and NH 3 gases, UF 4 is formed. In the highest valency, uranium exhibits amphoteric properties. Under the action of U0 3 acids or its hydrates, uranyl salts (U0 2 2+) are formed, colored yellow-green:

Most uranyl salts are highly soluble in water.

With alkalis, when fused, U0 3 forms salts of uranic acid - uranates MDKH,:

With alkaline solutions, uranium trioxide forms salts of polyuranic acids - polyuranates dgM 2 0y110 3 pH^O.

Salts of uranium acid are practically insoluble in water.

The acidic properties of U(VI) are less pronounced than the basic ones.

Uranium reacts with fluorine at room temperature. The stability of higher halides decreases from fluorides to iodides. Fluorides UF 3 , U4F17, U2F9 and UF 4 are non-volatile, and UFe is volatile. The most important of the fluorides are UF 4 and UFe.

Ftpppippyanir okgilya t "yanya ppptrkart in practice:

The reaction in a fluidized bed is carried out according to the equation:

It is possible to use fluorinating agents: BrF 3, CC1 3 F (freon-11) or CC1 2 F 2 (freon-12):

Uranium (1U) fluoride UF 4 ("green salt") - powder from bluish-green to emerald color. G 11L \u003d SW6 °; G to, ",. \u003d -1730 °. DYa ° 29 8 = 1856 kJ / mol. The crystal structure is monoclinic (sp. gp C2/c; 0=1.273 nm; 5=1.075 nm; 0=0.843 nm; d= 6.7 nm; p \u003d 12b ° 20 "; density 6.72 g / cm3. UF 4 is a stable, inactive, non-volatile compound, poorly soluble in water. The best solvent for UF 4 is fuming perchloric acid HC10 4. It dissolves in oxidizing acids to form a uranyl salt quickly dissolves in a hot solution of Al(N0 3) 3 or A1C1 3 , as well as in a solution of boric acid acidified with H 2 S0 4 , HC10 4 or HC1. or boric acid, also contribute to the dissolution of UF 4. Forms a number of sparingly soluble double salts with fluorides of other metals (MeUFe, Me 2 UF6, Me 3 UF 7, etc.) NH 4 UF 5 is of industrial importance.

U(IV) fluoride is an intermediate product in the preparation

both UF6 and uranium metal.

UF 4 can be obtained by reactions:

or by electrolytic reduction of uranyl fluoride.

Uranium hexafluoride UFe - at room temperature, ivory crystals with a high refractive index. Density

5.09 g/cm3, density of liquid UFe is 3.63 g/cm3. Flying connection. Tvoag = 5^>5°> Gil=64.5° (under pressure). Saturated vapor pressure reaches the atmosphere at 560°. Enthalpy of formation of AR° 29 8 = -2116 kJ/mol. The crystal structure is rhombic (sp. gr. Rpta; 0=0.999 nm; fe= 0.8962 nm; c=0.5207 nm; d 5.060 nm (250). MPC - 0.015 mg / m3. From the solid state, UF6 can sublime from the solid phase (sublimate) into a gas, bypassing the liquid phase over a wide range of pressures. Heat of sublimation at 50 0 50 kJ/mg. The molecule does not have a dipole moment, so UF6 does not associate. Vapors UFr, - an ideal gas.

It is obtained by the action of fluorine on U of its compounds:

In addition to gas-phase reactions, there are also liquid-phase reactions.

obtaining UF6 using halofluorides, for example

There is a way to obtain UF6 without the use of fluorine - by oxidizing UF 4:

UFe does not react with dry air, oxygen, nitrogen and CO 2, but upon contact with water, even with traces of it, it undergoes hydrolysis:

It interacts with most metals, forming their fluorides, which complicates the methods of its storage. Suitable vessel materials for working with UF6 are: Ni, Monel and Pt when heated, Teflon, absolutely dry quartz and glass, copper and aluminum when cold. At temperatures of 25 yuo 0 it forms complex compounds with fluorides of alkali metals and silver of the type 3NaFUFr>, 3KF2UF6.

It dissolves well in various organic liquids, inorganic acids and in all halogen fluorides. Inert to dry 0 2 , N 2 , CO 2 , C1 2 , Br 2 . UFr is characterized by reduction reactions with most pure metals. UF6 reacts vigorously with hydrocarbons and other organic substances, so closed containers of UFe can explode. UF6 in the range 25 - 100° forms complex salts with fluorides of alkali and other metals. This property is used in technology for the selective extraction of UF

Uranium hydrides UH 2 and UH 3 occupy an intermediate position between salt-like hydrides and hydrides such as solid solutions of hydrogen in metal.

When uranium reacts with nitrogen, nitrides are formed. IN U-N system four phases are known: UN (uranium nitride), a-U 2 N 3 (sesquinitride), p-U 2 N 3 and UN If90. It is not possible to reach the composition of UN 2 (dinitride). Reliable and well controlled are the syntheses of uranium mononitride UN, which are best done directly from the elements. Uranium nitrides are powdery substances, the color of which varies from dark gray to gray; look like metal. UN has a cubic face-centered crystal structure, such as NaCl (0=4.8892 A); (/ = 14.324, 7 ^ = 2855 °, stable in vacuum up to 1700 0. It is obtained by reacting U or U hydride with N 2 or NH 3 , decomposition of higher nitrides U at 1300 ° or their reduction with metallic uranium. U 2 N 3 is known in two polymorphic modifications: cubic a and hexagonal p (0=0.3688 nm, 6=0.5839 nm), releases N 2 in vacuum above 8oo°. It is obtained by reduction of UN 2 with hydrogen. Dinitride UN 2 is synthesized by the reaction of U with N 2 at high pressure N 2 . Uranium nitrides are readily soluble in acids and alkali solutions, but decompose with molten alkalis.

Uranium nitride is obtained by two-stage carbothermal reduction of uranium oxide:

Heating in argon at 7M450 0 for 10 * 20 hours

It is possible to obtain uranium nitride with a composition close to dinitride, UN 2 , by the action of ammonia on UF 4 at high temperature and pressure.

Uranium dinitride decomposes when heated:

Uranium nitride, enriched in 235U, has a higher fission density, thermal conductivity and melting point than uranium oxides, the traditional fuel of modern power reactors. It also has good mechanical and stability, exceeding traditional fuel. Therefore, this compound is considered as a promising basis for nuclear fuel fast neutron reactors (generation IV nuclear reactors).

Comment. UN is very useful to enrich on ‘5N, because ,4 N tends to capture neutrons, generating the radioactive isotope 14 C by the (n, p) reaction.

Uranium carbide UC 2 (?-phase) - light gray with a metallic sheen crystalline substance. IN U-C system(uranium carbides) there are UC 2 (?-phase), UC 2 (b 2-phase), U 2 C 3 (e-phase), UC (b 2-phase) - uranium carbides. Uranium dicarbide UC 2 can be obtained by the reactions:

U + 2C ^ UC 2 (54v)

Uranium carbides are used as fuel for nuclear reactors, they are promising as fuel for space rocket engines.

Uranyl nitrate, uranyl nitrate, U0 2 (N0 3) 2 -6H 2 0. The role of the metal in this salt is played by the uranyl cation 2+. Yellow crystals with a greenish sheen, easily soluble in water. The aqueous solution is acidic. Soluble in ethanol, acetone and ether, insoluble in benzene, toluene and chloroform. When heated, the crystals melt and release HN0 3 and H 2 0. The crystalline hydrate easily erodes in air. A characteristic reaction is that under the action of NH 3 a yellow precipitate of ammonium urate is formed.

Uranium is able to form metal organic compounds. Examples are cyclopentadienyl derivatives of the composition U(C 5 H 5) 4 and their halogenated u(C 5 H 5) 3 G or u(C 5 H 5) 2 G 2 .

IN aqueous solutions uranium is most stable in the oxidation state U(VI) in the form of the uranyl ion U0 2 2+ . To a lesser extent, it is characterized by the U(IV) state, but it can even exist in the U(III) form. The U(V) oxidation state can exist as the IO 2 + ion, but this state is rarely observed due to the tendency to disproportionation and hydrolysis.

In neutral and acidic solutions, U(VI) exists as U0 2 2+ - a yellow uranyl ion. Well-soluble uranyl salts include nitrate U0 2 (N0 3) 2, sulfate U0 2 S0 4, chloride U0 2 C1 2, fluoride U0 2 F 2, acetate U0 2 (CH 3 C00) 2. These salts are isolated from solutions in the form of crystalline hydrates with different numbers of water molecules. Slightly soluble salts of uranyl are: oxalate U0 2 C 2 0 4, phosphates U0 2 HP0., and UO2P2O4, ammonium uranyl phosphate UO2NH4PO4, sodium uranyl vanadate NaU0 2 V0 4, ferrocyanide (U0 2) 2. The uranyl ion has a tendency to form complex compounds. So complexes with fluorine ions of the type -, 4- are known; nitrate complexes ‘ and 2 *; sulfate complexes 2 "and 4-; carbonate complexes 4" and 2 ", etc. Under the action of alkalis on solutions of uranyl salts, sparingly soluble precipitates of diuranates of the Me 2 U 2 0 7 type are released (Me 2 U0 4 monouranates are not isolated from solutions, they are obtained by fusion uranium oxides with alkalis) Me 2 U n 0 3 n+i polyuranates are known (for example, Na 2 U60i 9).

U(VI) is reduced in acidic solutions to U(IV) by iron, zinc, aluminum, sodium hydrosulfite, and sodium amalgam. The solutions are colored green. Alkalis precipitate from them hydroxide and 0 2 (0H) 2, hydrofluoric acid - fluoride UF 4 -2.5H 2 0, oxalic acid - oxalate U (C 2 0 4) 2 -6H 2 0. The tendency to complex formation in the U 4+ ion less than that of uranium ions.

Uranium (IV) in solution is in the form of U 4+ ions, which are highly hydrolyzed and hydrated:

Hydrolysis is inhibited in acidic solutions.

Uranium (VI) in solution forms uranyl oxocation - U0 2 2+ Numerous uranyl compounds are known, examples of which are: U0 3, U0 2 (C 2 H 3 0 2) 2, U0 2 C0 3 -2 (NH 4) 2 C0 3 U0 2 C0 3 , U0 2 C1 2 , U0 2 (0H) 2 , U0 2 (N0 3) 2 , UO0SO4, ZnU0 2 (CH 3 C00) 4 etc.

During the hydrolysis of the uranyl ion, a number of multinuclear complexes are formed:

With further hydrolysis, U 3 0s (0H) 2 appears and then U 3 0 8 (0H) 4 2 -.

For the qualitative detection of uranium, methods of chemical, luminescent, radiometric and spectral analyzes are used. Chemical Methods predominantly based on the formation of colored compounds (for example, red-brown color of the compound with ferrocyanide, yellow with hydrogen peroxide, blue with the arsenazo reagent). The luminescent method is based on the ability of many uranium compounds to give a yellowish-greenish glow under the action of UV rays.

Quantitative determination of uranium is carried out by various methods. The most important of them are: volumetric methods, consisting in the reduction of U(VI) to U(IV) followed by titration with solutions of oxidizing agents; weight methods - precipitation of uranates, peroxide, U(IV) kupferranates, oxyquinolate, oxalate, etc. followed by their calcination at 100° and weighing U 3 0s; polarographic methods in a nitrate solution make it possible to determine 10 x 7 x 10-9 g of uranium; numerous colorimetric methods (for example, with H 2 0 2 in an alkaline medium, with the arsenazo reagent in the presence of EDTA, with dibenzoylmethane, in the form of a thiocyanate complex, etc.); luminescent method, which makes it possible to determine when fused with NaF to yu 11 g uranium.

235U belongs to group A of radiation hazard, the minimum significant activity MZA=3.7-10 4 Bq, 2 s 8 and - to group D, MZA=3.7-10 6 Bq (300 g).

URANUS (the name in honor of the planet Uranus discovered shortly before him; lat. uranium * a. uranium; n. Uran; f. uranium; and. uranio), U, - radioactive chemical element III group of the periodic system Mendeleev, atomic number 92, atomic mass 238.0289, refers to actinides. Natural uranium consists of a mixture of three isotopes: 238 U (99.282%, T 1/2 4.468.10 9 years), 235 U (0.712%, T 1/2 0.704.10 9 years), 234 U (0.006%, T 1/2 0.244.10 6 years). 11 artificial radioactive isotopes of uranium with mass numbers from 227 to 240 are also known.

Uranium was discovered in 1789 in the form of UO 2 by the German chemist M. G. Klaproth. Metallic uranium was obtained in 1841 by the French chemist E. Peligot. For a long time, uranium had a very limited use, and only with the discovery of radioactivity in 1896 did its study and use begin.

Properties of uranium

In the free state, uranium is a light gray metal; below 667.7°C, it is characterized by a rhombic (a=0.28538 nm, b=0.58662 nm, c=0.49557 nm) crystal lattice (a-modification), in the temperature range 667.7-774°C - tetragonal (a = 1.0759 nm, c = 0.5656 nm; R-modification), at a higher temperature - body-centered cubic lattice (a = 0.3538 nm, g-modification). Density 18700 kg / m 3, melting t 1135 ° C, boiling t about 3818 ° C, molar heat capacity 27.66 J / (mol.K), specific electrical resistance 29.0.10 -4 (Ohm.m), thermal conductivity 22.5 W/(m.K), temperature coefficient of linear expansion 10.7.10 -6 K -1 . The transition temperature of uranium to the superconducting state is 0.68 K; weak paramagnetic, specific magnetic susceptibility 1.72.10 -6 . The nuclei 235 U and 233 U fission spontaneously, as well as during the capture of slow and fast neutrons, 238 U fissions only during the capture of fast (more than 1 MeV) neutrons. When slow neutrons are captured, 238 U turns into 239 Pu. The critical mass of uranium (93.5% 235U) in aqueous solutions is less than 1 kg, for an open ball about 50 kg; for 233 U the critical mass is approximately 1/3 of the critical mass of 235 U.

Education and content in nature

The main consumer of uranium is nuclear power engineering (nuclear reactors, nuclear power plants). In addition, uranium is used to produce nuclear weapons. All other fields of uranium use are of sharply subordinate importance.

Uranium (U) is an element with atomic number 92 and atomic weight 238.029. It is a radioactive chemical element of the III group of the periodic system of Dmitry Ivanovich Mendeleev, belongs to the family of actinides. Uranium is a very heavy (2.5 times heavier than iron, more than 1.5 times heavier than lead), silvery-white glossy metal. In its pure form, it is slightly softer than steel, malleable, flexible, and has slight paramagnetic properties.

Natural uranium consists of a mixture of three isotopes: 238U (99.274%) with a half-life of 4.51∙109 years; 235U (0.702%) with a half-life of 7.13∙108 years; 234U (0.006%) with a half-life of 2.48∙105 years. The last isotope is not primary, but radiogenic; it is part of the 238U radioactive series. 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.

Currently, 23 artificial radioactive isotopes of uranium with mass numbers from 217 to 242 are known. Among them, 233U with a half-life of 1.62∙105 years is the longest-lived one. It is obtained as a result of neutron irradiation of thorium, capable of fission under the influence of thermal neutrons.

Uranium was discovered in 1789 by the German chemist Martin Heinrich Klaproth as a result of his experiments with the mineral pitchblende. The name of the new element was in honor of the recently discovered (1781) planet Uranus by William Herschel. For the next half century, the substance obtained by Klaproth was considered a metal, but in 1841 this was refuted by the French chemist Eugene Melchior Peligot, who proved the oxide nature of uranium (UO2) obtained by the German chemist. Peligo himself managed to obtain metallic uranium by reducing UCl4 with metallic potassium, as well as to determine the atomic weight of the new element. The next in the development of knowledge about uranium and its properties was D. I. Mendeleev - in 1874, based on the theory he developed about the periodization of chemical elements, he placed uranium in the farthest cell of his table. The atomic weight of uranium (120) previously determined by Peligo was doubled by the Russian chemist, the correctness of such assumptions was confirmed twelve years later by the experiments of the German chemist Zimmermann.

For many decades, uranium was of interest only to a narrow circle of chemists and natural scientists, its use was also limited - the production of glass and paints. Only with the discovery of the radioactivity of this metal (in 1896 by Henri Becquerel) did the industrial processing of uranium ores begin in 1898. Much later (1939) the phenomenon of nuclear fission was discovered, and since 1942 uranium has become the main nuclear fuel.

The most important property of uranium is that the nuclei of some of its isotopes are capable of fission when they capture neutrons, as a result of this process, a huge amount of energy is released. This property of element No. 92 is used in nuclear reactors that serve as energy sources, and also underlies the action atomic bomb. Uranium is used in geology to determine the age of minerals and rocks in order to clarify the sequence of flow geological processes(geochronology). Due to the fact that rocks contain different concentrations of uranium, they have different radioactivity. This property is used in the selection of rocks by geophysical methods. This method is most widely used in petroleum geology for well logging. Uranium compounds were used as paints for painting on porcelain and for ceramic glazes and enamels (colored in colors: yellow, brown, green and black, depending on the degree of oxidation), for example, sodium uranate Na2U2O7 was used as a yellow pigment in painting.

Biological properties

Uranium is a fairly common element in the biological environment; certain types of fungi and algae are considered to be concentrators of this metal, which are included in the chain of the biological cycle of uranium in nature according to the scheme: water - aquatic plants - fish - man. Thus, with food and water, uranium enters the body of humans and animals, and more precisely into gastrointestinal tract, where about a percent of the incoming readily soluble compounds and no more than 0.1% of sparingly soluble compounds are absorbed. In the respiratory tract and lungs, as well as in the mucous membranes and skin, this element enters with air. In the respiratory tract, and especially the lungs, absorption is much more intense: easily soluble compounds are absorbed by 50%, and sparingly soluble by 20%. Thus, uranium is found in small amounts (10-5 - 10-8%) in the tissues of animals and humans. In plants (in the dry residue), the concentration of uranium depends on its content in the soil, so at a soil concentration of 10-4%, the plant contains 1.5∙10-5% or less. The distribution of uranium in tissues and organs is uneven, the main places of accumulation are bone tissues (skeleton), liver, spleen, kidneys, as well as lungs and broncho-pulmonary lymph nodes (when sparingly soluble compounds enter the lungs). Uranium (carbonates and complexes with proteins) is quickly eliminated from the blood. On average, the content of the 92nd element in the organs and tissues of animals and humans is 10-7%. For example, the blood of cattle contains 1∙10-8 g/ml of uranium, while human blood contains 4∙10-10 g/g. Cattle liver contains 8∙10-8 g/g, in humans in the same organ 6∙10-9 g/g; the spleen of cattle contains 9∙10-8 g/g, in humans - 4.7∙10-7 g/g. In the muscle tissues of cattle, it accumulates up to 4∙10-11 g/g. In addition, in the human body, uranium is contained in the lungs in the range of 6∙10-9 - 9∙10-9 g/g; in the kidneys 5.3∙10-9 g/g (cortical layer) and 1.3∙10-8 g/g (medulla); V bone tissue 1∙10-9 g/g; in the bone marrow 1∙10-8 g/g; in hair 1.3∙10-7 g/g. The uranium in the bones causes constant irradiation of bone tissue (the period of complete removal of uranium from the skeleton is 600 days). Least of all this metal in the brain and heart (about 10-10 g / g). As mentioned earlier, the main ways in which uranium enters the body are water, food and air. The daily dose of metal entering the body with food and liquids is 1.9∙10-6 g, with air - 7∙10-9 g. However, every day uranium is excreted from the body: with urine from 0.5∙10-7 g up to 5∙10-7 g; with feces from 1.4∙10-6 g to 1.8∙10-6 g. Losses with hair, nails and dead skin flakes - 2∙10-8 g.

Scientists suggest that uranium in scanty amounts is necessary for the normal functioning of the human body, animals and plants. However, its role in physiology has not yet been elucidated. It has been established that the average content of the 92nd element in the human body is about 9∙10-5 g (International Commission on Radiation Protection). True, this figure varies somewhat for different regions and territories.

Despite its as yet unknown, but certain biological role in living organisms, uranium remains one of the most dangerous elements. First of all, this is manifested in the toxic effect of this metal, which is due to its chemical properties, in particular on the solubility of the compounds. So, for example, soluble compounds (uranyl and others) are more toxic. Most often, poisoning with uranium and its compounds occurs at enrichment plants, enterprises for the extraction and processing of uranium raw materials, and other production facilities where uranium is involved in technological processes.

Penetrating into the body, uranium affects absolutely all organs and their tissues, because the action occurs at the cell level: it inhibits the activity of enzymes. The kidneys are primarily affected, which manifests itself in a sharp increase in sugar and protein in the urine, subsequently developing oliguria. The gastrointestinal tract and liver are affected. Uranium poisoning is divided into acute and chronic, the latter developing gradually and may be asymptomatic or with mild manifestations. However, later chronic poisoning leads to hematopoietic disorders, nervous system and other serious health problems.

A ton of granite rock contains approximately 25 grams of uranium. The energy that can be released during the combustion of these 25 grams in a reactor is comparable to the energy that is released during the combustion of 125 tons of coal in the furnaces of powerful thermal boilers! Based on these data, it can be assumed that in the near future granite will be considered one of the types of mineral fuel. In total, the relatively thin twenty-kilometer surface layer of the earth's crust contains approximately 1014 tons of uranium, when converted into an energy equivalent, a simply colossal figure is obtained - 2.36.1024 kilowatt-hours. Even all the developed, explored and prospective deposits of combustible minerals taken together are not capable of providing even a millionth of this energy!

It is known that uranium alloys subjected to heat treatment are characterized by high yield strength, creep and increased corrosion resistance, less propensity to change products under temperature fluctuations and under the influence of irradiation. Based on these principles, at the beginning of the 20th century and up to the thirties, uranium in the form of carbide was used in the production of tool steels. In addition, he went to replace tungsten in some alloys, which was cheaper and more accessible. In the production of ferrouranium, the share of U was up to 30%. True, in the second third of the 20th century, such use of uranium came to naught.

As you know, in the bowels of our Earth there is a constant process of decay of urn isotopes. So, scientists have calculated that the instantaneous release of the energy of the entire mass of this metal, enclosed in the earth's shell, would warm up our planet to a temperature of several thousand degrees! However, such a phenomenon, fortunately, is impossible - after all, heat is released gradually - as the nuclei of uranium and its derivatives undergo a series of long-term radioactive transformations. The duration of such transformations can be judged from the half-lives of natural uranium isotopes, for example, for 235U it is 7108 years, and for 238U - 4.51109 years. However, uranium heat significantly warms the Earth. If there were as much uranium in the entire mass of the Earth as in the upper twenty-kilometer layer, then the temperature on the planet would be much higher than now. However, as one moves toward the center of the Earth, the concentration of uranium decreases.

In nuclear reactors, only a small part of the loaded uranium is processed, this is due to the slagging of the fuel with fission products: 235U burns out, the chain reaction gradually fades. However, fuel rods are still filled with nuclear fuel, which must be reused. To do this, the old fuel elements are dismantled and sent for processing - they are dissolved in acids, and the uranium is extracted from the resulting solution by extraction, the fission fragments that need to be disposed of remain in the solution. Thus, it turns out that the uranium industry is practically waste-free chemical production!

Plants for the separation of uranium isotopes occupy an area of ​​several tens of hectares, approximately the same order of magnitude and the area of ​​porous partitions in the separation cascades of the plant. This is due to the complexity of the diffusion method for separating uranium isotopes - after all, in order to increase the concentration of 235U from 0.72 to 99%, several thousand diffusion steps are needed!

Using the uranium-lead method, geologists managed to find out the age of the most ancient minerals, while studying meteorite rocks, they managed to determine the approximate date of the birth of our planet. Thanks to the "uranium clock" determined the age of the lunar soil. Interestingly, it turned out that for 3 billion years there has been no volcanic activity on the Moon and natural satellite The earth remains a passive body. After all, even the youngest pieces of lunar matter have lived longer than the age of the most ancient terrestrial minerals.

Story

The use of uranium began a very long time ago - as early as the 1st century BC, natural uranium oxide was used to make a yellow glaze used in the coloring of ceramics.

In modern times, the study of uranium proceeded gradually - in several stages, with a continuous increase. The beginning was the discovery of this element in 1789 by the German natural philosopher and chemist Martin Heinrich Klaproth, who restored the golden-yellow “earth” mined from Saxon resin ore (“uranium pitch”) to a black metal-like substance (uranium oxide - UO2). The name was given in honor of the most distant planet known at that time - Uranus, which in turn was discovered in 1781 by William Herschel. At this, the first stage in the study of a new element (Klaproth was sure that he had discovered a new metal) ends, there comes a break of more than fifty years.

The year 1840 can be considered the beginning of a new milestone in the history of uranium research. It was from this year that a young chemist from France, Eugene Melchior Peligot (1811-1890), took up the problem of obtaining metallic uranium, soon (1841) he succeeded - metallic uranium was obtained by reducing UCl4 with metallic potassium. In addition, he proved that the uranium discovered by Klaproth was in fact just its oxide. The Frenchman also determined the estimated atomic weight of the new element - 120. Then again there is a long break in the study of the properties of uranium.

Only in 1874 new assumptions about the nature of uranium appear: Dmitry Ivanovich Mendeleev, following the theory he developed on the periodization of chemical elements, finds a place for a new metal in his table, placing uranium in the last cell. In addition, Mendeleev increases the previously assumed atomic weight of uranium by two, without making a mistake in this either, which was confirmed by the experiments of the German chemist Zimmermann 12 years later.

Since 1896, discoveries in the field of studying the properties of uranium “fell down” one after another: in the year mentioned above, quite by accident (when studying the phosphorescence of potassium uranyl sulfate crystals), 43-year-old physics professor Antoine Henri Becquerel discovers Becquerel Rays, later renamed radioactivity by Marie Curie . In the same year, Henri Moissan (again a chemist from France) develops a method for obtaining pure metallic uranium.

In 1899, Ernest Rutherford discovered the inhomogeneity of the radiation of uranium preparations. It turned out that there are two types of radiation - alpha and beta rays, different in their properties: they carry different electric charge, have different path lengths in matter and their ionizing ability is also different. A year later, gamma rays were also discovered by Paul Villard.

Ernest Rutherford and Frederick Soddy jointly developed the theory of uranium radioactivity. Based on this theory, in 1907 Rutherford undertook the first experiments to determine the age of minerals in the study of radioactive uranium and thorium. In 1913, F. Soddy introduced the concept of isotopes (from the ancient Greek iso - “equal”, “same”, and topos - “place”). In 1920, the same scientist suggested that isotopes could be used to determine the geological age of rocks. His assumptions turned out to be correct: in 1939, Alfred Otto Karl Nier created the first equations for calculating age and used a mass spectrometer to separate isotopes.

In 1934, Enrico Fermi conducted a series of experiments on the bombardment of chemical elements with neutrons - particles discovered by J. Chadwick in 1932. As a result of this operation, previously unknown radioactive substances appeared in uranium. Fermi and other scientists who participated in his experiments suggested that they had discovered transuranium elements. For four years, attempts were made to detect transuranium elements among the products of neutron bombardment. It all ended in 1938, when the German chemists Otto Hahn and Fritz Strassmann found that, capturing a free neutron, the nucleus of the 235U uranium isotope is divided, while a sufficiently large energy is released (per one uranium nucleus), mainly due to kinetic energy fragments and radiation. To advance further, the German chemists failed. Lisa Meitner and Otto Frisch were able to substantiate their theory. This discovery was the origin of the use of intra-atomic energy, both for peaceful and military purposes.

Being in nature

The average content of uranium in the earth's crust (clarke) is 3∙10-4% by mass, which means that it is more in the bowels of the earth than silver, mercury, bismuth. Uranium is a characteristic element for the granite layer and sedimentary shell of the earth's crust. So, in a ton of granite there is about 25 grams of element No. 92. In total, more than 1000 tons of uranium are contained in the relatively thin, twenty-kilometer, upper layer of the Earth. In acid igneous rocks 3.5∙10-4%, in clays and shales 3.2∙10-4%, especially enriched in organic matter, in basic rocks 5∙10-5%, in ultrabasic rocks of the mantle 3∙10-7% .

Uranium migrates vigorously in cold and hot, neutral and alkaline waters in the form of simple and complex ions, especially in the form of carbonate complexes. An important role in the geochemistry of uranium is played by redox reactions, all because uranium compounds, as a rule, are highly soluble in waters with an oxidizing environment and poorly soluble in waters with a reducing environment (hydrogen sulfide).

More than a hundred mineral ores of uranium are known, they are different in chemical composition, origin, concentration of uranium, of the whole variety, only a dozen are of practical interest. The main representatives of uranium, which have the greatest industrial importance, in nature can be considered oxides - uraninite and its varieties (nasturan and uranium black), as well as silicates - coffinite, titanates - davidite and brannerite; aqueous phosphates and uranyl arsenates - uranium mica.

Uraninite - UO2 is present mainly in the ancient - Precambrian rocks in the form of clear crystalline forms. Uraninite forms isomorphic series with thorianite ThO2 and yttrocerianite (Y,Ce)O2. In addition, all uraninites contain radiogenic decay products of uranium and thorium: K, Po, He, Ac, Pb, as well as Ca and Zn. Uraninite itself is a high-temperature mineral, characteristic of granite and syenite pegmatites in association with complex uranium niob-tantalum-titanates (columbite, pyrochlore, samarskite, and others), zircon, and monazite. In addition, uraninite occurs in hydrothermal, skarn, and sedimentary rocks. Large deposits of uraninite are known in Canada, Africa, the United States of America, France and Australia.

Nasturan (U3O8), also known as uranium pitch or resin blende, which forms cryptocrystalline collomorphic aggregates, is a volcanogenic and hydrothermal mineral, present in Paleozoic and younger high- and medium-temperature formations. The constant companions of pitchblende are sulfides, arsenides, native bismuth, arsenic and silver, carbonates and some other elements. These ores are very rich in uranium, but extremely rare, often accompanied by radium, this is easily explained: radium is a direct product of the isotopic decay of uranium.

Uranium blacks (loose earthy aggregates) are mainly represented in young - Cenozoic and younger formations, characteristic of hydrothermal uranium sulfide and sedimentary deposits.

Uranium is also extracted as a by-product from ores containing less than 0.1%, for example, from gold-bearing conglomerates.

The main deposits of uranium ores are located in the USA (Colorado, North and South Dakota), Canada (provinces of Ontario and Saskatchewan), South Africa (Witwatersrand), France (Central Massif), Australia (Northern Territory) and many other countries. In Russia, the main uranium-ore region is Transbaikalia. About 93% of Russian uranium is mined at the deposit in the Chita region (near the city of Krasnokamensk).

Application

Modern nuclear energy is simply unthinkable without element No. 92 and its properties. Although not so long ago - before the launch of the first nuclear reactor, uranium ores were mined mainly to extract radium from them. Small amounts of uranium compounds have been used in some dyes and catalysts. In fact, uranium was considered an element of almost no industrial value, and how dramatically the situation changed after the discovery of the ability of uranium isotopes to fission! This metal instantly received the status of strategic raw material No. 1.

Nowadays, the main field of application of metallic uranium, as well as its compounds, is fuel for nuclear reactors. So, in stationary reactors of nuclear power plants, a low-enriched (natural) mixture of uranium isotopes is used, and uranium of a high degree of enrichment is used in nuclear power plants and fast neutron reactors.

The uranium isotope 235U has the greatest application, because it is possible for a self-sustaining nuclear chain reaction, which is not typical for other uranium isotopes. Thanks to this property, 235U is used as a fuel in nuclear reactors, as well as in nuclear weapons. However, the isolation of the 235U isotope from natural uranium is a complex and costly technological problem.

The most abundant uranium isotope in nature, 238U, can fission when bombarded with high-energy neutrons. This property of this isotope is used to increase the power of thermonuclear weapons - neutrons generated by a thermonuclear reaction are used. In addition, the plutonium isotope 239Pu is obtained from the 238U isotope, which in turn can also be used in nuclear reactors and in the atomic bomb.

IN Lately The isotope of uranium 233U artificially obtained in reactors from thorium is of great use; it is obtained by irradiating thorium in the neutron flux of a nuclear reactor:

23290Th + 10n → 23390Th -(β–)→ 23391Pa –(β–)→ 23392U

233U is fissioned by thermal neutrons, in addition, expanded reproduction of nuclear fuel can occur in reactors with 233U. So, when a kilogram of 233U burns out in a thorium reactor, 1.1 kg of new 233U should accumulate in it (as a result of the capture of neutrons by thorium nuclei). In the near future, the uranium-thorium cycle in thermal neutron reactors is the main competitor of the uranium-plutonium cycle for breeding nuclear fuel in fast neutron reactors. Reactors using this nuclide as fuel already exist and operate (KAMINI in India). 233U is also the most promising fuel for gas-phase nuclear rocket engines.

Other artificial uranium isotopes do not play a significant role.

After the “necessary” isotopes 234U and 235U are extracted from natural uranium, the remaining raw material (238U) is called “depleted uranium”, it is half as radioactive as natural uranium, mainly due to the removal of 234U from it. Since the main use of uranium is energy production, for this reason, depleted uranium is a low-use product with low economic value. However, due to its low price, as well as its high density and extremely high capture cross section, it is used for radiation shielding, and as ballast in aerospace applications such as control surfaces. aircraft. In addition, depleted uranium is used as ballast in space descent vehicles and racing yachts; in high speed gyroscope rotors, large flywheels, oil drilling.

However, the best-known use of depleted uranium is its use in military applications - as cores for armor-piercing projectiles and modern tank armor, such as the M-1 Abrams tank.

Lesser known applications of uranium are mainly associated with its compounds. So a small addition of uranium gives a beautiful yellow-green fluorescence to glass, some uranium compounds are photosensitive, for this reason uranyl nitrate was widely used to enhance negatives and stain (tint) positives (photographic prints) brown.

Carbide 235U alloyed with niobium carbide and zirconium carbide is used as fuel for nuclear jet engines. Alloys of iron and depleted uranium (238U) are used as powerful magnetostrictive materials. Sodium uranate Na2U2O7 was used as a yellow pigment in painting; previously, uranium compounds were used as paints for painting on porcelain and for ceramic glazes and enamels (colored in colors: yellow, brown, green and black, depending on the degree of oxidation).

Production

Uranium is obtained from uranium ores, which differ significantly in a number of characteristics (according to the conditions of formation, by "contrast", by the content of useful impurities, etc.), the main of which is the percentage of uranium. According to this feature, five grades of ores are distinguished: very rich (contain over 1% uranium); rich (1-0.5%); medium (0.5-0.25%); ordinary (0.25-0.1%) and poor (less than 0.1%). However, even from ores containing 0.01-0.015% uranium, this metal is extracted as a by-product.

Over the years of development of uranium raw materials, many methods have been developed for separating uranium from ores. This is due both to the strategic importance of uranium in some areas, and to the diversity of its natural manifestations. However, despite all the variety of methods and raw material base, any uranium production consists of three stages: preliminary concentration of uranium ore; leaching of uranium and obtaining sufficiently pure uranium compounds by precipitation, extraction or ion exchange. Further, depending on the purpose of the resulting uranium, the enrichment of the product with the 235U isotope follows, or immediately the reduction of elemental uranium.

So, initially the ore is concentrated - the rock is crushed and filled with water. In this case, the heavier elements of the mixture precipitate faster. In rocks containing primary uranium minerals, their rapid precipitation occurs, since they are very heavy. When concentrating ores containing secondary uranium minerals, sedimentation of waste rock occurs, which is much heavier than secondary minerals, but can contain very useful elements.

Uranium ores are almost not enriched, except for the organic method of radiometric sorting, based on the γ-radiation of radium, which always accompanies uranium.

The next step in uranium production is leaching, so the uranium goes into solution. Basically, ores are leached with solutions of sulfuric, sometimes nitric acids or soda solutions with the transfer of uranium into an acidic solution in the form of UO2SO4 or complex anions, and into a soda solution in the form of a 4-complex anion. The method in which sulfuric acid is used is cheaper, however, it is not always applicable - if the raw material contains tetravalent uranium (uranium resin), which does not dissolve in sulfuric acid. In such cases, alkaline leaching is used or the tetravalent uranium is oxidized to the hexavalent state. The use of caustic soda (caustic soda) is useful when leaching ore containing magnesite or dolomite, which require too much acid to dissolve.

After the leaching stage, the solution contains not only uranium, but also other elements, which, like uranium, are extracted with the same organic solvents, precipitate on the same ion-exchange resins, and precipitate under the same conditions. In such a situation, for the selective separation of uranium, one has to use many redox reactions in order to exclude an undesirable element at different stages. One of the advantages of ion exchange and extraction methods is that uranium is quite completely extracted from poor solutions.

After all these operations, uranium is transferred to a solid state - into one of the oxides or into UF4 tetrafluoride. Such uranium contains impurities with a large thermal neutron capture cross section - lithium, boron, cadmium, and rare earth metals. In the final product, their content should not exceed hundred-thousandths and millionths of a percent! To do this, the uranium is dissolved again, this time in nitric acid. Uranyl nitrate UO2(NO3)2 during extraction with tributyl phosphate and some other substances is additionally purified to the required conditions. This substance is then crystallized (or precipitated) and gently ignited. As a result of this operation, uranium trioxide UO3 is formed, which is reduced with hydrogen to UO2. At temperatures from 430 to 600 ° C, uranium oxide reacts with dry hydrogen fluoride and turns into UF4 tetrafluoride. Already from this compound, metallic uranium is usually obtained with the help of calcium or magnesium by conventional reduction.

Physical properties

Metallic uranium is very heavy, it is two and a half times heavier than iron, and one and a half times heavier than lead! This is one of the heaviest elements that are stored in the bowels of the Earth. With its silvery-white color and brilliance, uranium resembles steel. pure metal plastic, soft, has a high density, but at the same time it is easy to process. Uranium is electropositive, has insignificant paramagnetic properties - the specific magnetic susceptibility at room temperature is 1.72 10 -6, has low electrical conductivity, but high reactivity. This element has three allotropic modifications: α, β and γ. The α-form has a rhombic crystal lattice with the following parameters: a = 2.8538 Å, b = 5.8662 Å, c = 469557 Å. This form is stable in the temperature range from room temperature to 667.7°C. The density of uranium in the α-form at 25°C is 19.05±0.2 g/cm 3 . The β-form has a tetragonal crystal lattice, is stable in the temperature range from 667.7° C to 774.8° C. The parameters of the quadrangular lattice: a = 10.759 Å, b = 5.656 Å. γ-form with body-centered cubic structure, stable from 774.8°C to melting point (1132°C).

You can see all three phases in the process of uranium reduction. For this, a special apparatus is used, which is a seamless steel pipe, which is lined with calcium oxide, it is necessary that the steel of the pipe does not interact with uranium. A mixture of uranium and magnesium (or calcium) tetrafluoride is loaded into the apparatus, after which it is heated to 600 ° C. When this temperature is reached, an electric fuse is turned on, it instantly flows exothermic reduction reaction, while the loaded mixture completely melts. Liquid uranium (temperature 1132 ° C) due to its weight completely sinks to the bottom. After complete deposition of uranium on the bottom of the apparatus, cooling begins, uranium crystallizes, its atoms line up in a strict order, forming a cubic lattice - this is the γ-phase. The next transition occurs at 774°C - the crystal lattice of the cooling metal becomes tetragonal, which corresponds to the β-phase. When the temperature of the ingot drops to 668° C, the atoms rearrange their rows again, arranged in waves in parallel layers - the α-phase. There are no further changes.

The main parameters of uranium always refer to the α-phase. Melting point (tmelt) 1132°C, boiling point of uranium (tboil) 3818°C. Specific heat at room temperature 27.67 kJ/(kg K) or 6.612 cal/(g°C). The specific electrical resistance at a temperature of 25 ° C is approximately 3 10 -7 ohm cm, and already at 600 ° C 5.5 10 -7 ohm cm. The thermal conductivity of uranium also varies depending on temperature: for example, in the range of 100-200 ° C, it is 28.05 W / (m K) or 0.067 cal / (cm sec ° C), and when it rises to 400 ° C, it increases up to 29.72 W / (m K) 0.071 cal / (cm sec ° C). Uranium has superconductivity at 0.68 K. The average Brinell hardness is 19.6 - 21.6·10 2 MN / m 2 or 200-220 kgf / mm 2.

Many mechanical properties of the 92nd element depend on its purity, on the modes of thermal and mechanical processing. So for cast uranium ultimate tensile strength at room temperature 372-470 MN/m 2 or 38-48 kgf/mm 2 , the average value of the modulus of elasticity 20.5·10 -2 MN/m2 or 20.9·10 -3 kgf/mm 2 . The strength of uranium increases after quenching from β- and γ-phases.

Irradiation of uranium with a neutron flux, interaction with water that cools fuel elements made of metallic uranium, and other factors of operation in powerful thermal neutron reactors - all this leads to changes in the physical and mechanical properties of uranium: the metal becomes brittle, creep develops, deformation of products from metallic uranium occurs . For this reason, uranium alloys are used in nuclear reactors, for example, with molybdenum, such an alloy is resistant to water, strengthens the metal, while maintaining a high-temperature cubic lattice.

Chemical properties

Chemically, uranium is a very active metal. In air, it oxidizes with the formation of an iridescent film of UO2 dioxide on the surface, which does not protect the metal from further oxidation, as happens with titanium, zirconium and a number of other metals. With oxygen, uranium forms UO2 dioxide, UO3 trioxide and a large number of intermediate oxides, the most important of which is U3O8, these oxides are similar in properties to UO2 and UO3. In the powdered state, uranium is pyrophoric and can ignite with slight heating (150 ° C and above), combustion is accompanied by a bright flame, eventually forming U3O8. At a temperature of 500-600 ° C, uranium interacts with fluorine to form green needle-shaped crystals that are slightly soluble in water and acids - uranium tetrafluoride UF4, as well as UF6 - hexafluoride (white crystals that sublimate without melting at a temperature of 56.4 ° C). UF4, UF6 are examples of the interaction of uranium with halogens to form uranium halides. Uranium combines easily with sulfur to form a range of compounds, of which highest value has US - nuclear fuel. Uranium reacts with hydrogen at 220°C to form UH3 hydride, which is chemically very active. Upon further heating, UH3 decomposes into hydrogen and powdered uranium. Interaction with nitrogen occurs at higher temperatures - from 450 to 700 °C and atmospheric pressure nitride U4N7 is obtained, with an increase in nitrogen pressure at the same temperatures, UN, U2N3 and UN2 can be obtained. At higher temperatures (750-800 °C), uranium reacts with carbon to form monocarbide UC, dicarbide UC2, and U2C3. Uranium interacts with water to form UO2 and H2, more slowly with cold water and more actively with hot water. In addition, the reaction proceeds with steam at temperatures from 150 to 250 °C. This metal dissolves in hydrochloric HCl and nitric HNO3 acids, less actively in highly concentrated hydrofluoric acid, slowly reacts with sulfuric H2SO4 and orthophosphoric H3PO4 acids. The products of reactions with acids are tetravalent salts of uranium. From inorganic acids and salts of some metals (gold, platinum, copper, silver, tin and mercury), uranium is able to displace hydrogen. Uranium does not interact with alkalis.

In compounds, uranium is able to exhibit the following oxidation states: +3, +4, +5, +6, sometimes +2. U3+ in natural conditions does not exist and can only be obtained in the laboratory. Pentavalent uranium compounds are for the most part unstable and decompose fairly easily into quaternary and hexavalent uranium compounds, which are the most stable. Hexavalent uranium is characterized by the formation of the uranyl ion UO22+, the salts of which are colored in yellow and are highly soluble in water and mineral acids. An example of compounds of hexavalent uranium is uranium trioxide or uranium anhydride UO3 (orange powder), which has the character of an amphoteric oxide. When dissolved in acids, salts are formed, for example, uranium chloride UO2Cl2. Under the action of alkalis on solutions of uranyl salts, salts of uranic acid H2UO4 - uranates and diuranic acid H2U2O7 - diuranates are obtained, for example, sodium uranate Na2UO4 and sodium diuranate Na2U2O7. Quadrivalent uranium salts (uranium tetrachloride UCl4) are green and less soluble. When exposed to air for a long time, compounds containing tetravalent uranium are usually unstable and turn into hexavalent ones. Uranyl salts such as uranyl chloride decompose in the presence of bright light or organics.