uranium (chemical element) uranium (chemical element)

URANIUM (lat. Uranium), U (read "uranium"), a radioactive chemical element with atomic number 92, atomic mass 238.0289. Actinoid. Natural uranium consists of a mixture of three isotopes: 238U, 99.2739%, with a half-life of T 1/2 \u003d 4.51 10 9 years, 235 U, 0.7024%, with a half-life T 1/2 \u003d 7.13 10 8 years, 234 U, 0.0057%, with a half-life T 1/2 = 2.45 10 5 years. 238 U (uranium-I, UI) and 235 U (actinouranium, AcU) are the founders of the radioactive series. Of the 11 artificially produced radionuclides with mass numbers 227-240, long-lived 233 U ( T 1/2 \u003d 1.62 10 5 years), it is obtained by neutron irradiation of thorium (cm. THORIUM).
Configuration of three outer electron layers 5 s 2 p 6 d 10 f 3 6s 2 p 6 d 1 7 s 2 , uranium refers to f-elements. It is located in IIIB group in the 7th period of the Periodic Table of the Elements. In compounds, it exhibits oxidation states +2, +3, +4, +5 and +6, valencies II, III, IV, V and VI.
The radius of the neutral atom of uranium is 0.156 nm, the radius of the ions: U 3 + - 0.1024 nm, U 4 + - 0.089 nm, U 5 + - 0.088 nm and U 6+ - 0.083 nm. The energies of successive ionization of an atom are 6.19, 11.6, 19.8, 36.7 eV. Electronegativity according to Pauling (cm. PAULING Linus) 1,22.
Discovery history
Uranium was discovered in 1789 by the German chemist M. G. Klaproth (cm. KLAPROT Martin Heinrich) in the study of the mineral "tar blende". Named after the planet Uranus, discovered by W. Herschel (cm. HERSHEL) in 1781. In the metallic state, uranium was obtained in 1841 by the French chemist E. Peligot (cm. PELIGO Eugene Melchior) when reducing UCl 4 with metallic potassium. The radioactive properties of uranium were discovered in 1896 by the Frenchman A. Becquerel (cm. Becquerel Antoine Henri).
Initially, uranium was assigned an atomic mass of 116, but in 1871 D. I. Mendeleev (cm. MENDELEEV Dmitry Ivanovich) came to the conclusion that it should be doubled. After the discovery of elements with atomic numbers from 90 to 103, the American chemist G. Seaborg (cm. SEABORG Glenn Theodore) came to the conclusion that these elements (actinides) (cm. actinoids) it is more correct to place in the periodic system in the same cell with element No. 89 actinium. This arrangement is due to the fact that actinides undergo completion of 5 f-electronic sublevel.
Being in nature
Uranium is a characteristic element for the granite layer and sedimentary shell of the earth's crust. The content in the earth's crust is 2.5 10 -4% by weight. In sea water, the concentration of uranium is less than 10 -9 g/l; in total, sea water contains from 10 9 to 10 10 tons of uranium. Uranium is not found in free form in the earth's crust. About 100 uranium minerals are known, the most important of them are pitchblende U 3 O 8, uraninite (cm. URANINITE)(U,Th)O 2, uranium resin ore (contains uranium oxides of variable composition) and tyuyamunite Ca[(UO 2) 2 (VO 4) 2] 8H 2 O.
Receipt
Uranium is obtained from uranium ores containing 0.05-0.5% U. The extraction of uranium begins with the production of a concentrate. Ores are leached with solutions of sulfuric, nitric acids or alkali. The resulting solution always contains impurities of other metals. When separating uranium from them, differences in their redox properties are used. Redox processes are combined with ion exchange and extraction processes.
From the resulting solution, uranium is extracted in the form of oxide or tetrafluoride UF 4 using the metallothermic method:
UF 4 + 2Mg = 2MgF 2 + U
The resulting uranium contains small amounts of boron impurities. (cm. BOR (chemical element)), cadmium (cm. CADMIUM) and some other elements, the so-called reactor poisons. By absorbing neutrons produced during the operation of a nuclear reactor, they make uranium unsuitable for use as a nuclear fuel.
To get rid of impurities, metallic uranium is dissolved in nitric acid, obtaining uranyl nitrate UO 2 (NO 3) 2 . The uranyl nitrate is extracted from the aqueous solution with tributyl phosphate. The purification product from the extract is again converted into uranium oxide or tetrafluoride, from which the metal is again obtained.
Part of the uranium is obtained by regeneration of spent nuclear fuel in the reactor. All uranium regeneration operations are carried out remotely.
Physical and chemical properties
Uranium is a silvery white lustrous metal. Uranium metal exists in three allotropic (cm. ALLOTROPY) modifications. Up to 669°C stable a-modification with an orthorhombic lattice, parameters but= 0.2854nm, in= 0.5869 nm and from\u003d 0.4956 nm, density 19.12 kg / dm 3. From 669°C to 776°C, the b-modification with a tetragonal lattice is stable (parameters but= 1.0758 nm, from= 0.5656 nm). Up to a melting point of 1135°C, the g-modification with a cubic body-centered lattice is stable ( but= 0.3525 nm). Boiling point 4200°C.
The chemical activity of metallic uranium is high. In air, it is covered with an oxide film. Powdered uranium is pyrophoric; during the combustion of uranium and the thermal decomposition of many of its compounds in air, uranium oxide U 3 O 8 is formed. If this oxide is heated in an atmosphere of hydrogen (cm. HYDROGEN) at temperatures above 500 ° C, uranium dioxide UO 2 is formed:
U 3 O 8 + H 2 \u003d 3UO 2 + 2H 2 O
If uranyl nitrate UO 2 (NO 3) 2 is heated at 500°C, then, decomposing, it forms uranium trioxide UO 3 . In addition to uranium oxides of the stoichiometric composition UO 2 , UO 3 and U 3 O 8 , uranium oxide of the composition U 4 O 9 and several metastable oxides and oxides of variable composition are known.
When uranium oxides are fused with oxides of other metals, uranates are formed: K 2 UO 4 (potassium uranate), CaUO 4 (calcium uranate), Na 2 U 2 O 7 (sodium diuranate).
Interacting with halogens (cm. HALOGENS), uranium gives uranium halides. Among them, UF 6 hexafluoride is a yellow crystalline substance that is easily sublimated even at low heating (40-60°C) and is equally easily hydrolyzed by water. The most important practical value is uranium hexafluoride UF 6 . It is obtained by the interaction of metallic uranium, uranium oxides or UF 4 with fluorine or fluorinating agents BrF 3 , CCl 3 F (freon-11) or CCl 2 F 2 (freon-12):
U 3 O 8 + 6CCl 2 F 2 = UF 4 + 3COCl 2 + CCl 4 + Cl 2
UF 4 + F 2 = UF 6
or
U 3 O 8 + 9F 2 \u003d 3UF 6 + 4O 2
Fluorides and chlorides are known that correspond to the oxidation states of uranium +3, +4, +5 and +6. Uranium bromides UBr 3 , UBr 4 and UBr 5 , as well as uranium iodides UI 3 and UI 4 were obtained. Uranium oxyhalides such as UO 2 Cl 2 UOCl 2 and others have been synthesized.
When uranium interacts with hydrogen, uranium hydride UH 3 is formed, which has a high chemical activity. When heated, the hydride decomposes, forming hydrogen and powdered uranium. During the sintering of uranium with boron, depending on the molar ratio of the reactants and the process conditions, borides UB 2 , UB 4 and UB 12 arise.
With carbon (cm. CARBON) uranium forms three carbides UC, U 2 C 3 and UC 2 .
The interaction of uranium with silicon (cm. SILICON) silicides U 3 Si, U 3 Si 2 , USi, U 3 Si 5 , USi 2 and U 3 Si 2 were obtained.
Uranium nitrides (UN, UN 2 , U 2 N 3) and uranium phosphides (UP, U 3 P 4 , UP 2) have been obtained. With sulfur (cm. SULFUR) uranium forms a series of sulfides: U 3 S 5 , US, US 2 , US 3 and U 2 S 3 .
Metallic uranium dissolves in HCl and HNO 3 and slowly reacts with H 2 SO 4 and H 3 PO 4 . There are salts containing the uranyl cation UO 2 2+ .
In aqueous solutions, there are uranium compounds in oxidation states from +3 to +6. Standard oxidation potential of U(IV)/U(III) pair - 0.52 V, U(V)/U(IV) pair 0.38 V, U(VI)/U(V) pair 0.17 V, pair U(VI)/U(IV) 0.27. The U 3+ ion is unstable in solution, the U 4+ ion is stable in the absence of air. The UO 2 + cation is unstable and disproportionates into U 4+ and UO 2 2+ in solution. U 3+ ions have a characteristic red color, U 4+ ions are green, and UO 2 2+ ions are yellow.
In solutions, uranium compounds in the +6 oxidation state are the most stable. All uranium compounds in solutions are prone to hydrolysis and complex formation, the most strongly are U 4+ and UO 2 2+ cations.
Application
Uranium metal and its compounds are mainly used as nuclear fuel in nuclear reactors. A low-enriched mixture of uranium isotopes is used in stationary reactors of nuclear power plants. The product of a high degree of enrichment is in nuclear reactors operating on fast neutrons. 235 U is the source of nuclear energy in nuclear weapons. 238 U serves as a source of secondary nuclear fuel - plutonium.
Physiological action
In microquantities (10 -5 -10 -8%) it is 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 -7 years.
Uranium and its compounds are highly toxic. Aerosols of uranium and its compounds are especially dangerous. For aerosols of water-soluble uranium compounds MPC in air is 0.015 mg/m 3 , for insoluble forms of uranium MPC is 0.075 mg/m 3 . When it enters the body, uranium acts on all organs, being a general cellular poison. 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.

encyclopedic Dictionary. 2009 .

See what "URANUS (chemical element)" is in other dictionaries:

U (Uran, uranium; at O ​​= 16 atomic weight U = 240) the element with the highest atomic weight; all elements, by atomic weight, are placed between hydrogen and uranium. This is the heaviest member of the metal subgroup of group VI of the periodic system (see Chromium, ... ... Encyclopedic Dictionary F.A. Brockhaus and I.A. Efron

Uranium (U) Atomic number 92 Appearance of a simple substance Properties of an atom Atomic mass ( molar mass) 238.0289 a. e.m. (g / mol) ... Wikipedia

Uranium (lat. Uranium), U, a radioactive chemical element of Group III of the Mendeleev periodic system, belongs to the actinide family, atomic number 92, atomic mass 238.029; metal. Natural U. consists of a mixture of three isotopes: 238U √ 99.2739% ... ... Great Soviet Encyclopedia

Uranium (chemical element)- URANIUM (Uranium), U, radioactive chemical element of group III of the periodic system, atomic number 92, atomic mass 238.0289; refers to actinides; metal, mp 1135°C. Uranium is the main element of nuclear energy (nuclear fuel), used in ... ... Illustrated Encyclopedic Dictionary Wikipedia

- (Greek uranos sky). 1) the god of heaven, the father of Saturn, the oldest of the gods, in Greek. mythol. 2) a rare metal that has the appearance of silvery leaves in its pure state. 3) a large planet discovered by Herschel in 1781. Dictionary of foreign words included in ... ... Dictionary of foreign words of the Russian language

Uranus:* Uranus (mythology) ancient Greek god. Son of Gaia * Uranus (planet) planet of the solar system * Uranus (musical instrument) ancient Turkic and Kazakh musical wind instrument * Uranus (element) chemical element * Operation ... ... Wikipedia

- (Uranium), U, radioactive chemical element of group III of the periodic system, atomic number 92, atomic mass 238.0289; refers to actinides; metal, mp 1135shC. Uranium is the main element of nuclear energy (nuclear fuel), used in ... ... Modern Encyclopedia

(according to Pauling) 1.38 U←U 4+ -1.38V
U←U 3+ -1.66V
U←U 2+ -0.1V 6, 5, 4, 3 Thermodynamic properties 19.05 / ³ 0.115 /( ) 27.5 /( ) 1405.5 12.6 / 4018 417 / 12.5³/ Crystal cell orthorhombic 2.850 c/a ratio n/a n/a

History

Even in ancient times (1st century BC), natural uranium was used to make yellow glaze for.

Uranium was discovered in 1789 by the German chemist Martin Heinrich Klaproth (Klaproth) while studying the mineral ("uranium tar"). It was named after it, discovered in 1781. In the metallic state, uranium was obtained in 1841 by the French chemist Eugene Peligot during the reduction of UCl 4 with metallic potassium. uranium was discovered in 1896 by a Frenchman. Initially, 116 was attributed to uranium, but in 1871 he came to the conclusion that it should be doubled. After the discovery of elements with atomic numbers from 90 to 103, the American chemist G. Seaborg came to the conclusion that it is more correct to place these elements () in the periodic system in the same cell with element No. 89. This arrangement is due to the fact that the 5f electron sublevel is completed in actinides.

Being in nature

Uranium is a characteristic element for the granite layer and sedimentary shell of the earth's crust. Content in the earth's crust 2.5 10 -4% by weight. In sea water, the concentration of uranium is less than 10 -9 g/l; in total, sea water contains from 10 9 to 10 10 tons of uranium. Uranium is not found in free form in the earth's crust. About 100 uranium minerals are known, the most important of them are U 3 O 8, uraninite (U,Th)O 2, uranium resin ore (contains uranium oxides of variable composition) and tuyamunite Ca [(UO 2) 2 (VO 4) 2] 8H 2 Oh

isotopes

Natural Uranium consists of a mixture of three isotopes: 238 U - 99.2739%, half-life T 1 / 2 = 4.51-10 9 years, 235 U - 0.7024% (T 1 / 2 = 7.13-10 8 years) and 234 U - 0.0057% (T 1 / 2 \u003d 2.48 × 10 5 years).

There are 11 known artificial radioactive isotopes with mass numbers from 227 to 240.

The most long-lived - 233 U (T 1 / 2 \u003d 1.62 10 5 years) is obtained by irradiating thorium with neutrons.

The uranium isotopes 238 U and 235 U are the progenitors of two radioactive series.

Receipt

The very first stage of uranium production is concentration. The rock is crushed and mixed with water. Heavy suspended matter components precipitate faster. If the rock contains primary uranium minerals, they precipitate quickly: these are heavy minerals. The secondary minerals of element #92 are lighter, in this case the 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 element No. 92 into solution. Apply acid and alkaline leaching. The first is cheaper, since uranium is used to extract. But if in the feedstock, as, for example, in uranium tar, uranium is in the tetravalent state, then this method is not applicable: tetravalent uranium in sulfuric acid is practically insoluble. And either you need to resort to alkaline leaching, or pre-oxidize uranium to a hexavalent state.

Do not use acid leaching and in cases where the uranium concentrate contains or. Too much acid has to be spent on dissolving them, and in these cases it is better to use ().

The problem of uranium leaching from is solved by oxygen purge. A stream is fed into a mixture of uranium ore and minerals heated to 150 °C. At the same time, it is formed from sulfurous minerals, which washes out uranium.

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

The solution contains not only uranium, but also others. Some of them under certain conditions behave in the same way as uranium: they are extracted with the same 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 quite fully extract uranium from poor solutions, in a liter of which there are only tenths of a gram of element No. 92.

After these operations, uranium is transferred to a solid state - into one of the oxides or into tetrafluoride UF 4 . But this uranium still needs to be purified from impurities with a large thermal neutron capture cross section - , . Their content in the final product should not exceed hundred thousandths and millionths of a percent. So the already obtained technically pure product has to be dissolved again - this time in. Uranyl nitrate UO 2 (NO 3) 2 during extraction with tributyl phosphate and some other substances is additionally purified to the desired conditions. Then this substance is crystallized (or precipitated peroxide UO 4 ·2H 2 O) and begin to carefully ignite. As a result of this operation, uranium trioxide UO 3 is formed, which is reduced to UO 2 .

This substance is the penultimate one on the way from ore to metal. At temperatures from 430 to 600 ° C, it reacts with dry hydrogen fluoride and turns into UF 4 tetrafluoride. It is from this compound that metallic uranium is usually obtained. Receive with the help or usual.

Physical properties

Uranium is a very heavy, silvery-white, shiny metal. In its pure form, it is slightly softer than steel, malleable, flexible, and has slight paramagnetic properties. Uranium has three allotropic forms: alpha (prismatic, stable up to 667.7 °C), beta (quadrangular, stable from 667.7 to 774.8 °C), gamma (with a body-centered cubic structure, existing from 774.8 °C to the melting point).

Chemical properties

The chemical activity of metallic uranium is high. In the air, it becomes covered with an iridescent film. Powdered uranium, it ignites spontaneously at a temperature of 150-175 °C. During the combustion of uranium and the thermal decomposition of many of its compounds in air, uranium oxide U 3 O 8 is formed. If this oxide is heated in the atmosphere at temperatures above 500 °C, UO 2 is formed. When uranium oxides are fused with oxides of other metals, uranates are formed: K 2 UO 4 (potassium uranate), CaUO 4 (calcium uranate), Na 2 U 2 O 7 (sodium diuranate).

Application

Nuclear fuel

Uranium 235 U has the greatest application, in which self-sustaining is possible. Therefore, this isotope is used as a fuel in, as well as in (critical mass of about 48 kg). Isolation of the isotope U 235 from natural uranium is a complex technological problem (see). The isotope U 238 is capable of fission under the influence of bombardment with high-energy neutrons, this feature is used to increase power (neutrons generated by a thermonuclear reaction are used). As a result of neutron capture followed by β-decay, 238 U can turn into 239 , which is then used as nuclear fuel.

Uranium-233 artificially obtained in reactors (by irradiation with neutrons and turning into and then into uranium-233) is nuclear fuel for nuclear power plants and the production of atomic bombs (critical mass of about 16 kg). Uranium-233 is also the most promising fuel for gas-phase nuclear rocket engines.

Other applications

• A small addition of uranium gives a beautiful greenish-yellow tint to the glass.
• Uranium-235 carbide in an alloy with niobium carbide and zirconium carbide is used as a fuel for nuclear jet engines (the working fluid is hydrogen + hexane).
• Alloys of iron and depleted uranium (uranium-238) are used as powerful magnetostrictive materials.
• At the beginning of the twentieth century uranyl nitrate has been widely used as a virating agent to produce tinted photographic prints.

depleted uranium

After extracting U-235 from natural uranium, the remaining material is called "depleted uranium" because it is depleted in the 235th isotope. According to some reports, about 560,000 tons of depleted uranium hexafluoride (UF 6) are stored in the United States. Depleted uranium is half as radioactive as natural uranium, mainly due to the removal of U-234 from it. Because the main use of uranium is energy production, depleted uranium is a useless product with little economic value.

Its main use is associated with the high density of uranium and its relatively low cost: its use for radiation protection (strange as it may seem) and as ballast in aerospace applications such as aircraft control surfaces. Each 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.

Armor-piercing projectile cores

The best-known use of uranium is as cores for American . Upon fusion with 2% or 0.75% and heat treatment (rapid quenching of metal heated to 850 °C in water or oil, further holding at 450 °C for 5 hours), metallic uranium becomes harder and stronger (tensile strength is more than 1600 MPa, while that for pure uranium it is 450 MPa). Combined with its high density, this makes hardened uranium ingot an extremely effective armor penetration tool, similar in effectiveness to the more expensive . The process of destruction of the armor is accompanied by grinding the uranium blank into dust and igniting it in air on the other side of the armor. About 300 tons of depleted uranium remained on the battlefield during Operation Desert Storm (mostly the remains of shells from the 30 mm GAU-8 cannon of A-10 attack aircraft, each shell contains 272 g of uranium alloy).

Such shells were used by NATO troops in the fighting in Yugoslavia. After their application, the ecological problem of radiation contamination of the country's territory was discussed.

Depleted uranium is used in modern tank armor, such as the tank.

Physiological action

In microquantities (10 -5 -10 -8%) it is 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, and broncho-pulmonary. The content in organs and tissues of humans and animals does not exceed 10 -7 g.

Uranium and its compounds toxic. Aerosols of uranium and its compounds are especially dangerous. For aerosols of water-soluble uranium compounds MPC in air is 0.015 mg/m 3 , for insoluble forms of uranium 0.075 mg/m 3 . When it enters the body, uranium acts on all organs, being a general cellular poison. The molecular mechanism of action of uranium is related to its ability to suppress activity. First of all, they are affected (protein and sugar appear in the urine,). In chronic cases, disorders of the hematopoiesis and nervous system are possible.

Uranium mining in the world

According to the "Red Book of Uranium", released in 2005, 41,250 tons of uranium were mined (in 2003 - 35,492 tons). According to the OECD, there are 440 commercial uses in the world that 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

Production in Russia

The remaining 7% is obtained by underground leaching of CJSC Dalur () and OJSC Khiagda ().

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

see also

Links

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 restrict ourselves to a brief description of the properties of the main fissile isotopes - uranium and plutonium.

Uranus

Uranus ( uranium) U - an element of the actinide group, 7th-0th period of the 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 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 nuclei 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 zzi (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 saves 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 232U, 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 a high level of purity for light elements is required for gun schemes. 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 lends itself 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. The elevated level of 234i 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 a much larger amount of 238U (with a cross section of 2.7 barn) is converted into 239Pu. 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 2 35U isotope 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. one.

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 spent 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, pU, stable from 667.7 to 774.8°), gamma (with a cubic body-centered lattice, yU, 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 from 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 physicochemical 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, but = 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 acids 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 126 ° 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. Four phases are known in the U-N system: 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 2 35U, 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) is a light gray crystalline substance with a metallic sheen. In the 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 is characterized by 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 hydroxide and 0 2 (0H) 2 from them, 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 uranyl ions.

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

Hydrolysis is suppressed 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 are mainly based on the formation of colored compounds (for example, red-brown color of the compound with ferrocyanide, yellow with hydrogen peroxide, blue with 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).

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Uranus is the seventh planet in the solar system and the third gas giant. The planet is the third largest and the fourth largest by mass, and received its name in honor of the father of the Roman god Saturn.

Exactly Uranus honored to be the first planet discovered in modern history. However, in reality, his original discovery of it as a planet did not actually happen. In 1781 the astronomer William Herschel when observing the stars in the constellation of Gemini, he noticed some disk-shaped object, which he first recorded in the category of comets, which he reported to the Royal Scientific Society of England. However, later Herschel himself was puzzled by the fact that the orbit of the object turned out to be practically circular, and not elliptical, as is the case with comets. And only when this observation was confirmed by other astronomers, Herschel came to the conclusion that he had actually discovered a planet, not a comet, and the discovery finally received wide recognition.

After confirming the data that the discovered object is a planet, Herschel received an unusual privilege - to give it his name. Without hesitation, the astronomer chose the name of the King of England George III and named the planet Georgium Sidus, which means "George's Star". However, the name never received scientific recognition and scientists, for the most part, came to the conclusion that it is better to adhere to a certain tradition in the name of the planets of the solar system, namely, to name them in honor of the ancient Roman gods. This is how Uranus got its modern name.

Currently, the only planetary mission that has been able to collect data on Uranus is Voyager 2.

This meeting, which took place in 1986, allowed scientists to obtain a fairly large amount of data about the planet and make many discoveries. The spacecraft transmitted thousands of photographs of Uranus, its moons and rings. Although many photographs of the planet showed little more than a blue-green color that could also be observed from ground-based telescopes, other images showed the presence of ten previously unknown satellites and two new rings. No new missions to Uranus are planned for the near future.

Due to the dark blue color of Uranus, it turned out to be much more difficult to make an atmospheric model of the planet than models of the same or even. Fortunately, images taken from the Hubble Space Telescope have provided a broader picture. More modern telescope imaging technologies made it possible to obtain much more detailed images than those of Voyager 2. So, thanks to the Hubble photographs, it was possible to find out that there are latitudinal bands on Uranus, like on other gas giants. In addition, the speed of the winds on the planet can reach over 576 km / h.

It is believed that the reason for the appearance of a monotonous atmosphere is the composition of its uppermost layer. Visible cloud layers are composed primarily of methane, which absorbs these observed red wavelengths. The reflected waves are thus represented as blue and green.

Beneath this outer layer of methane, the atmosphere is about 83% hydrogen (H2) and 15% helium, with some methane and acetylene present. This composition is similar to other gas giants of the solar system. However, the atmosphere of Uranus differs sharply in another respect. While the atmospheres of Jupiter and Saturn are mostly gaseous, the atmosphere of Uranus contains much more ice. Evidence of this are extremely low temperatures on the surface. Given the fact that the temperature of the atmosphere of Uranus reaches -224 ° C, it can be called the coldest of the atmospheres in the solar system. In addition, the available data indicate that such extremely low temperatures are present around almost the entire surface of Uranus, even on the side that is not illuminated by the Sun.

Uranus, according to planetary scientists, consists of two layers: the core and the mantle. Current models suggest that the core is mostly composed of rock and ice, and has about 55 times its mass. The mantle of the planet weighs 8.01 x 10 to the power of 24 kg, or about 13.4 Earth masses. In addition, the mantle is composed of water, ammonia, and other volatile elements. The main difference between the mantle of Uranus and Jupiter and Saturn is that it is icy, albeit not in the traditional sense of the word. The fact is that the ice is very hot and thick, and the thickness of the mantle is 5.111 km.

What's most amazing about Uranus' composition, and what sets it apart from other gas giants in our star system, is that it doesn't radiate more energy than it receives from the Sun. Considering the fact that even, which is very close in size to Uranus, it produces about 2.6 times more heat than it receives from the Sun, scientists today are very intrigued by such a weak power generated by Uranus. There are currently two explanations for this phenomenon. The first indicates that Uranus was impacted by a large space object in the past, which led to the loss of most of the planet's internal heat (gained during formation) into outer space. The second theory claims that there is a barrier inside the planet that does not allow the internal heat of the planet to escape to the surface.

Orbit and rotation of Uranus

The very discovery of Uranus allowed scientists to expand the radius of the known solar system by almost two times. This means that the average orbit of Uranus is about 2.87 x 10 to the power of 9 km. The reason for such a huge distance is the duration of the passage of solar radiation from the Sun to the planet. Sunlight takes about two hours and forty minutes to reach Uranus, which is almost twenty times longer than it takes sunlight to reach Earth. The huge distance also affects the length of the year on Uranus, it lasts almost 84 Earth years.

The orbital eccentricity of Uranus is 0.0473, which is only slightly less than that of Jupiter - 0.0484. This factor makes Uranus the fourth of all the planets in the solar system in terms of a circular orbit. The reason for such a small eccentricity of the orbit of Uranus is the difference between its perihelion of 2.74 x 10 to the power of 9 km and aphelion of 3.01 x 109 km is only 2.71 x 10 to the power of 8 km.

The most interesting moment in the process of rotation of Uranus is the position of the axis. The fact is that the axis of rotation for every planet except Uranus is roughly perpendicular to their orbital plane, however, Uranus's axis is tilted by almost 98°, which effectively means that Uranus rotates on its side. The result of this position of the planet's axis is that the north pole of Uranus is on the Sun for half of the planetary year, and the other half falls on the south pole of the planet. In other words, daytime on one hemisphere of Uranus lasts 42 Earth years, and night time on the other hemisphere lasts the same. The reason why Uranus "turned on its side", scientists again call a collision with a huge cosmic body.

Given the fact that the rings of Saturn were the most popular of the rings in our solar system for a long time, the rings of Uranus could not be detected until 1977. However, the reason is not only this, there are two more reasons for such a late discovery: the distance of the planet from the Earth and the low reflectivity of the rings themselves. In 1986, the Voyager 2 spacecraft was able to determine the presence of two more rings on the planet, in addition to those known at that time. In 2005, the Hubble Space Telescope spotted two more. To date, planetary scientists know 13 rings of Uranus, the brightest of which is the Epsilon ring.

The rings of Uranus differ from those of Saturn in almost everything - from particle size to composition. First, the particles that make up the rings of Saturn are small, little more than a few meters in diameter, while the rings of Uranus contain many bodies up to twenty meters in diameter. Second, the particles of Saturn's rings are mostly ice. The rings of Uranus, however, are composed of both ice and significant dust and debris.

William Herschel discovered Uranus only in 1781, as the planet was too dim to be seen by representatives of ancient civilizations. Herschel himself at first believed that Uranus was a comet, but later revised his opinion and science confirmed the planetary status of the object. So Uranus became the first planet discovered in modern history. The original name proposed by Herschel was "George's Star" - in honor of King George III, but the scientific community did not accept it. The name "Uranus" was proposed by the astronomer Johann Bode, in honor of the ancient Roman god Uranus.
Uranus rotates on its axis once every 17 hours and 14 minutes. Likewise, the planet rotates in a retrograde direction, opposite to the direction of the Earth and the other six planets.
It is believed that the unusual tilt of the axis of Uranus could cause a grandiose collision with another cosmic body. The theory is that the planet, which was supposedly the size of the Earth, collided sharply with Uranus, which shifted its axis by almost 90 degrees.
Wind speeds on Uranus can reach up to 900 km per hour.
The mass of Uranus is about 14.5 times that of the Earth, making it the lightest of the four gas giants in our solar system.
Uranus is often referred to as an "ice giant". In addition to hydrogen and helium in the upper layer (like other gas giants), Uranus also has an icy mantle that surrounds its iron core. The upper atmosphere is composed of ammonia and icy methane crystals, giving Uranus its characteristic pale blue color.
Uranus is the second least dense planet in the solar system, after Saturn.