Uranium is an element with an atomic number of 92. Its elemental symbol is U, the heaviest element found in nature. There are three isotopes in nature, all of which are radioactive and have very long half-lives (hundreds of thousands to 4.5 billion years). uranium uses

There are also 12 artificial isotopes (226U~240U). Uranium was discovered in 1789 by Martin Heinrich Klaproth. Uranium compounds were used early in the coloring of porcelain and were used as nuclear fuels after nuclear fission was discovered.

Uranium Element

The heaviest metal naturally produced. It is silvery-white and has the characteristics of strong hardness, high density, extensibility, and radioactivity. Uranium is generally found in the combination of uranium with oxygen, oxides or silicates.

Uranium atoms can undergo fission reactions and release a large amount of energy, which can be applied to power generation, nuclear weapons manufacturing, and other fields.

uranium 235

The nuclear weapons program of the Second World Great and Medium Allied Forces triggered demand for uranium, and uranium production came into being. By the 1970s, the uranium production industry had been firmly established.

Chemical Properties

Uranium is a radioactive chemical element of the III b family, with the symbol U, atomic number 92, and the relative atomic mass of 238.03, which is the natural element with the highest atomic number and relative atomic mass. Uranium is a silvery-white dense metal at room temperature.

The new cut surface of uranium is shiny steel gray, but a black oxide film is gradually formed in the air at room temperature.

The outer electron layer configuration of the uranium atom is [Rn] 5f3 6d1 7s2, and the 5f3 6d1 7s2 shell is a valence electron. Uranium has four valence states of +3, +4, +5, and +6, with +4 and +6 valences as the main.

Uranium is a positively charged, active element that reacts with almost all non-metallic elements (except inert gases) to form compounds, often in the form of U3+, U4+, UO2+, and UO22+ ions. Uranium and hydrogen react reversibly at 523 K to form UH3.  uranium uses

The uranium-oxygen system is relatively complex, and there are many phases between UO2 and UO3. The important oxides are UO2, U3 O8, and UO3. Among them, UO2 is currently the most widely used nuclear fuel. Uranium and halogen are important compounds in the preparation of nuclear fuels.

For example, UF4 is an intermediate product for the production of metallic uranium and UF6. The triple point of UF6 is 337K, which is the raw material for the separation of gaseous uranium isotopes. Uranium carbide, uranium nitride, and uranium silicide are all promising nuclear fuels with superior performance.

Metal uranium darkens in the air and can be corroded by steam and acid, but is resistant to alkali corrosion. The atomic radius is 138.5 pm, the ionic radii of U3+, U4+, U5+, and U6+ are 103, 97, 89, and 80 pm, respectively.

The electronegativity of uranium was determined to be 1.38 according to Pauling, Allred and Rochow were determined to be 1.22.

Uranium reacts with most non-metallic elements and their compounds. The temperature and reaction rate of the reaction varies with the particle size of the uranium.

reaction

Uranium can ignite spontaneously in air or oxygen at room temperature, and fine uranium can also ignite spontaneously in water. uranium uses

Under certain conditions, the energy released by uranium oxidation can cause an explosion. The lower limit of the explosive concentration of uranium dust is 55mg/dm3. Uranium reacts with many metals to form intermetallic compounds. Uranium can form solid solution with lanthanum, cerium, zirconium, molybdenum, and titanium.

Uranium and its compounds have large chemical toxicity. The allowable concentration of soluble uranium compounds in the air is 0.05mg/m3, and the allowable concentration of insoluble uranium compounds is 0.25mg/m3.

The radioactive allowable dose of natural uranium is soluble. The uranium compound is 7400Bq and the insoluble uranium compound is 333Bq.

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Physical Properties

Uranium is a radioactive metallic element that acts as a fuel for nuclear reactions. Uranium is a silver-white metal, almost as hard as steel, high in density (relative density about 18.95), melting point 1135 ° C, and boiling point 4134 °C. It was used to make yellow glass before the development of nuclear energy. Uranium is the most atomic element in nature.

In 1841, E. Paley (1811-1890) isolated metal uranium, although uranium was previously recognized in asphalt uranium.

It is also found in mica uranium, vanadium-potassium uranium and monazite; mainly distributed in Canada, Australia, and South Africa. The isotope uranium can be separated from the volatile gas uranium hexafluoride (UF6) by gas diffusion technology.

There are three allotropes in uranium, and their temperature and main structural characteristics are listed in the table.  uranium uses

The density of α-U at room temperature was 19.02 t/m3. α -U and β-U exhibit obvious anisotropy. For example, between 298 and 523 K, the thermal expansion coefficients of α-U single crystal along a, b and c axes are αa = +33.24×10 -6 /K, respectively, α b = -6.49 × 10 -6 /K, α c = + 30.36 × 10 -6 /K. γ-U has an isotropic structure.

The thermal expansion coefficient of the disordered polycrystalline uranium in the range of 293-373 K is equal to 16.3×10 -6 /K. The specific heat between 5 and 350 K is 27.66 J/(mol·K). The thermal conductivity of α-U increases with increasing temperature, 25.1 W/(m·K) at room temperature and 37.7 W/(m·K) at 1033 K.

The mechanical properties of uranium vary with the furnace number and heat treatment. For α-rolled α-annealed samples, the maximum yield strength at room temperature is 206.8-275.8 MPa, and the tensile strength of uranium is squeezed for small deformation.

The room temperature tensile strength limit is 586.1-861.8 MPa. Uranium has three crystal lattice structures: α-U is an orthorhombic structure, a=284.785pm, b=585.801pm, c=494.553pm; β-U is a square structure, a=1076.0pm, c=565.2pm, γ-U For body-centered cubic structure, a = 352.4 pm. Their switching temperatures are 941K (α→β) and 1047K (β→γ).

Isotope and its half life

Natural uranium contains three isotopes: 238U, 235U and 234U, which are 99.28% (238U), 0.71% (235U) and 0.006% (234U), respectively, with a half-life of (238U) 4.51. ×109, (235U) 7.09 × 108 and (234U) 2.35 × 105 years. Among them, 235U is the most important, and it is currently the fuel of nuclear power.

A 235U core absorbs a thermal neutron when fission occurs, releasing about 2.5 neutrons and releasing 207 MeV of energy. The energy released by 1 kg 235U nuclear fission is equivalent to the energy produced by burning 2700 t of coal.

Depending on the reactor type and its operating conditions, nuclear fuel can be either natural uranium or enriched uranium with increased U content.

Separation of uranium isotopes by gas diffusion, centrifugation or laser methods can achieve a U enrichment of more than 90%. U captures neutrons and transforms into fissile Pu. Pu and U are also the main raw materials for the manufacture of nuclear weapons. uranium uses

The 25km crust contains 1014 t of uranium, of which seawater contains 1010 t, and the average seawater per ton contains 3.3 mg of uranium. There are hundreds of minerals containing uranium in nature, but most of them are poor ore, so it is difficult to economically exploit large quantities.

isotopes of uranium

At present, the economically valuable uranium mine contains U3O8 in an amount of about 0.1%. If a fast neutron breeder is developed, the utilization of uranium resources can be increased by 60 to 70 times compared with pressurized water reactors.

The most abundant uranium isotope is 238U, and the other is 235U, which can be used as a fuel for nuclear power generation. The least abundant is 234U. There are also 12 artificial isotopes. uranium uses

Isotope

Abundance

Uranium

Half Life

Decay Mode

Decay

Energy (MeV)

Decay

Product

232 UArtificial68.9 yearsSpontaneous division
Alpha decay5.414Th-228
233 UArtificial159200Spontaneous division197.93
Alpha decay4.909Th-229
234 U0.006%245500 yearsSpontaneous division197.78
Alpha decay4.859Th-230
235 U0.72%7.038×10^8 yearsSpontaneous division202.48
Alpha decay4.679Th-231
236 UArtificial2.342×10^7 yearsSpontaneous division201.82
Alpha decay4.572Th-232
237 UArtificial6.75 daysBeta decay0.519Np-237
238 U99.275%4.468×10^9 yearsSpontaneous division205.87
Alpha decay4.270Th-234

 

Discover a Brief History

Uranium (yóu) English name Uranium, named after Uranus ‘s name “Uranus”. 1789 MH Klaproth (Martin Heinrich Klaproth) from pitchblende first discovered in a “U”, to use a 1781 newly discovered planet – Uranus named it uranium, element symbols for the U.

However, in 1841, Eugene-Melchior Peligot proved that the substance was uranium dioxide, followed by potassium reduction of UCl4 to prepare metal uranium.

In 1896, Antoine Henri Becquerel discovered the radioactivity of uranium. At that time, the study of uranium was purely theoretical, and uranium compounds were only used for the coloration of glass and ceramics. In 1898, radium was discovered in uranium mines, and uranium became a by-product of mining radium.

In 1938, Otto Hahn and Fritz Strassmann used neutrons to bombard the uranium nucleus and discovered nuclear fission while releasing energy, which caused people to re-emphasize uranium.

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During the Second World War and after the war, the exploration and exploitation of uranium resources was accelerated due to the needs of nuclear weapons and nuclear power.

The United States has established a special agency for the study of atomic bombs. In 1945, the United States through the first 239Pu atomic bomb in Hiroshima, Japan, and a few days later dropped a 235U atomic bomb in Nagasaki, Japan.  uranium uses

In 1954 the Soviet Union built its first nuclear power plant. Since then, the research and production of uranium has been highly valued by all countries in the world, and the nuclear weapons manufacturing and nuclear power generation industries have developed rapidly.

Uranium Alloy

Uranium can form intermetallic compounds with a variety of metals. Uranium has chemically active, anisotropic structures and poor mechanical properties.

Some properties of uranium alloys are superior to metal uranium, which is quite important in the manufacture of nuclear fuel components. Adding appropriate amounts of other metals, such as bismuth, chromium, molybdenum or zirconium, can improve the thermal conductivity, crystal structure and metallographic structure, heat treatment characteristics, irradiation stability and corrosion resistance of uranium.

uranium element

Depleted uranium bomb is a kind of high-efficiency combustion armor-piercing projectile made of depleted uranium alloy. uranium uses

It is made of high density (two times of lead density), high strength (three times of steel strength) and strong penetrating power, and is easy to burn, depleted uranium alloy containing 238U, 235U, etc., remaining after the explosion of depleted uranium 235U, etc. can damage the kidneys, nervous system, can cause lung cancer.

Depleted uranium alloys with high density and hardness can also be made of radiation-proof materials.

Use of Uranium

Before 1942, uranium was mainly used as a coloring agent for glass and ceramics in a small amount. With the discovery of the 235U chain nuclear fission reaction, the huge energy released by nuclear fission (1kg 235U released fission energy equivalent to 1800tTNT explosive) has attracted people’s attention, first used to make atomic bombs, hydrogen bombs.

Uranium nuclear reactor

Since the late 1950s, uranium has been increasingly used as a nuclear fuel for nuclear power generation. The energy released by 1kg 235U nuclear complete fission is equivalent to the energy released by burning 2700t of high quality coal. uranium uses

Nuclear Reaction

In addition, uranium nuclear reactors can also be used as sources of radiation for agricultural irradiation breeding, food industry food preservation and sterilization, and for the production of artificial elements.

In medicine, it is used in radiotherapy, radioimmunoassay kits, angiography, etc., in industrial and geological applications for industrial flaw detection, automatic control, geological exploration and archaeological archaeology.

Scientific research and industrial practice have proven that uranium is the only natural nuclear fuel and the nuclear industry must rely on uranium.

Since the nuclear energy industry has both purposes of peace and military application, uranium has become a special commodity metal whose production is influenced by various political, social and economic factors.

In the 1940s and 1950s, uranium was mainly used for nuclear weapons, and it was mainly used for nuclear power generation after the 1950s. The world’s uranium production has been oversupplied for a long time and has a large inventory. uranium uses

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The price per kilogram of U3 O8 in the international market has fallen from $97 in early 1978 to $19.84 in 1990. The annual production of uranium in Western countries also fell from 43960t in 1980 to 35278t in 1985.

However, during this period, nuclear power plants developed rapidly. In 1980, the total installed capacity was 135 million kW, and in 1989 it increased to 318 million kW. The annual production of uranium in 1985 was lower than the demand for nuclear power generation.

Atomic bomb

Regularly place conventional explosives around the uranium and then use electronic detonators to make these explosives accurate.

At the same time, the explosion, the enormous pressure generated, pressed the uranium together and was compressed to reach critical conditions and explode. Or if two pieces of uranium with a total mass exceeding the critical mass are brought together, a violent explosion will occur.

Atomic bomb

Critical mass refers to the quality of fissile material required to maintain a nuclear chain reaction. Different fissile materials are subject to different properties due to factors such as the nature of the nucleus (such as fission cross-section), physical properties, material shape, purity, whether it is surrounded by neutron reflective materials, whether there is neutron absorbing material, etc. quality.

A combination that happens to produce a chain reaction is called a critical point. More than this combination of masses, the rate of nuclear reactions will increase exponentially, called supercritical.

If the combination is capable of a chain reaction without delaying the release of the neutron, this threshold is called the immediate criticality and is a supercritical one. uranium uses

The critical combination will produce a nuclear explosion. If the combination is smaller than the critical point, the fission will decrease with time, which is called sub-critical.

Nuclear weapons must remain sub-critical before they detonate. Taking uranium nuclear bombs as an example, uranium can be divided into several large blocks, each of which is maintained below the critical mass. Quickly combine the uranium blocks during detonation.

The “Little Boy” atomic bomb thrown in Hiroshima is a small piece of uranium that is shot through a barrel to another large piece of uranium, resulting in sufficient quality. This design is called “gun type”.

Uranium Nuclear Fission

In nature, 234U does not undergo nuclear fission. Generally, 238U does not undergo fission. Only 235U is prone to nuclear fission, and nuclear fuel mainly refers to 235 U. The half-life of 235U is 7.038×108 years.

Starting from 235U, after 11 consecutive decays, a stable 207Pb appears. The half-life of 238U was 4.468 × 109 years. Starting from 238U, after 14 consecutive decays, the stable 206Pb-206 appeared. In 238U continuous decay, the longest half-life of the daughter nucleus is 234U, and its half-life is 2.45 × 105 years. uranium uses

235U, 233U and 239Pu are the main nuclear fission materials, which can be directly used as nuclear fuel. They can be obtained in large quantities and absorb slow neutrons (energy less than 1 eV) and undergo fission.

234U and 238U are not. 235U is found in natural uranium, and 233U and 239Pu are produced by uranium nuclear reactors. 235U, 233U and 239Pu, any energy neutron can cause them to split and release energy for 235U, the slower the neutron is, the more likely it is to cause fission. 238U absorbs a neutron and can also be converted into fissile material.

Both 235U and 238U can spontaneously fission, but the probability of spontaneous fission is small. uranium uses

235U fission

Studies have shown that after 235U absorbs slow neutrons, there are more than 40 types of fission, at least 300 kinds of nuclide and fast neutrons (average 2.5) of 36 elements, and release huge energy. In addition to neutrons, uranium nuclear fission products usually have two (two-split) fissures, three (three-split) and four fissiles the probability of “three divisions” is extremely small.

In addition to neutrons, there are many combinations of “two-split” fragments of uranium core. The mass ratio of fragments is roughly 3:2, and the chance of the same mass is very small.

The uranium “three divisions”, one of them it is an alpha particle, and the probability of occurrence of “three divisions” is 3/1000 of “two divisions”.

Statistics show that the neutron energy (kinetic energy) of the 235U fission emission is in the range of 0.1-20 MeV, with an average of 2 MeV.

Only fast neutrons cannot produce a continuous fission chain reaction of natural uranium, slow neutrons cannot fission 238U, and continuous fission reaction is unlikely to occur in 238U.

235U and 240Pu, etc., in addition to neutrons can cause their nuclear fission, charged particles or gamma rays with sufficient energy can also initiate fission.  uranium uses

In addition, uranium also produces trapping resonances for neutrons of about 25 eV, ie, capture without fission.

235U has small binding energy and low nuclear fission barrier. Any energy neutron can make it fission, and for slow neutrons (neutron rate is 2.2×10 3 m/s, which is equivalent to the rate of gas molecules moving at normal temperature).

In this way, it has a relatively long time near the uranium core and is easy to hit the uranium core to fission. It has a large fission cross section (the probability of fission is large).

235U absorbs a slow neutron, usually forming an excited state of 236U (review), then splitting into two, while releasing neutrons and energy.

In a thermal neutron (a type of slow neutron) reactor, the 235U thermal neutron fission cross section is 200 times larger than the 238U thermal neutron fission cross section, so that a sufficient number of neutrons cause 235U nuclear fission This can make up for the weaker 235U content in natural uranium or enriched uranium, the utilization rate of uranium in this reactor is 1%-2%.

238U fission

238U (240Pu, 232Th) fission is valved, neutrons less than 1.1 MeV will be absorbed or scattered by them, and will not cause fission, larger energy neutrons can make them fission, but the possibility is extremely small. 238U has a large binding energy, a high fission barrier, and a fast neutron with an energy exceeding 1.4 MeV can fission and release a large neutron energy.  uranium uses

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Studies have shown that 238U has many resonance absorption peaks above several MeV, and its fission probability increases with the increase of neutron energy. 238U is not susceptible to fission, but it can become a good nuclear fission material such as 239Pu and 233U after neutron absorption.

The probability of a thermal neutron being captured by 238U is about 1/190 of the probability that a thermal neutron will cause a 235U fission. The main role of fast neutrons with the 238U core is inelastic collisions. Most neutrons reduce energy through inelastic collisions and are absorbed by 238 U nuclei in multiple collisions.