What is Uranium?
Uranium is an element with an atomic number of 92, and its element symbol is U, which is the heaviest element that can be 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). In addition, there are 12 artificial isotopes (226U ~ 240U). Uranium was discovered in 1789 by Martin Heinrich Klaproth. Uranium compounds were used to color porcelain early, and were used as nuclear fuel after nuclear fission was discovered [1] .
- Radiation from uranium ore? More
- Uranium ore does have radiation, but it is terrible that it was not expected. Studies have shown that a person holding about 1 kilogram of uranium ore receives the same amount of radiation per day as wearing a luminous watch. Previously, uranium was used as a toner to make good-looking glass. This is uranium glass.
- Refining uranium is more difficult than gold rushing in sari
- In natural uranium, the content of uranium 235 is only 0.72%, and the content of uranium 238 is greater than 99.2%. The refining of uranium is to increase the concentration of uranium 235. Almost all of the natural uranium is uranium 238. It is too difficult to increase the concentration of uranium 235, because they belong to the same element, but the mass is only 3 neutrons apart. Their chemical properties are almost the same. of.
- Chinese name
- uranium
- Foreign name
- Uranium
- Element symbol
- U
- Atomic weight
- 238.02891
- Element type
- metal element
- Atomic number
- 92
- Find people
- Klaprot
- Uranium is an element with an atomic number of 92, and its element symbol is U, which is the heaviest element that can be 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). In addition, there are 12 artificial isotopes (226U ~ 240U). Uranium was discovered in 1789 by Martin Heinrich Klaproth. Uranium compounds were used to color porcelain early, and were used as nuclear fuel after nuclear fission was discovered [1] .
- The most dangerous substance on earth
- Calcareous mica
- Content collating action
Brief introduction of uranium
- The heaviest metal produced naturally. It is silver-white, with strong hardness, high density, ductility, and radioactivity. Uranium is generally found in the combination of uranium with oxygen, oxides or silicates. Uranium atom can undergo fission reaction and release a large amount of energy so that it can be used in power generation, nuclear weapon manufacturing and other fields. The Second World Coalition s nuclear weapons program triggered demand for uranium, and uranium production came into being. By the 1970s, the uranium production industry had been firmly established [2] .
Chemical properties of uranium
- Uranium is a Group III b actinide radiochemical element, symbol U, atomic number 92, relative atomic mass 238.03, and is the natural element with the largest atomic number and relative atomic mass [3] . Uranium is a dense silver-white metal at room temperature [4] , and the new section of uranium is a shiny steel gray, but a black oxide film is gradually formed in air at room temperature [3] .
- The configuration of the outer electron layer of the uranium atom is [Rn] 5f 3 6d 1 7s 2 , and the shell of 5f 3 6d 1 7s 2 is a valence electron. Uranium has four valence states of +3, +4, +5, and +6, with +4 and +6 valence states as the main [4] .
- Uranium is a very positively active element that reacts with almost all non-metallic elements (except inert gases) to produce compounds, which often exist in the form of U 3+ , U 4+ , UO 2 + and UO 2 2+ ions. Uranium and hydrogen react reversibly at 523K to form UH 3 . The uranium-oxygen system is relatively complicated. There are multiple phases between UO 2 and UO 3. The important oxides are UO 2 , U 3 O 8 and UO 3 . Among them, UO 2 is currently the most widely used nuclear fuel. Uranium and halogens are important compounds in the preparation of nuclear fuel. For example, UF 4 is an intermediate product for the production of metallic uranium and UF 6 . The triple point of UF 6 is 337K, which is the raw material for gaseous uranium isotope separation. Uranium carbide, uranium nitride, and uranium silicide are all considered promising nuclear fuels with superior performance [3] .
- Metal uranium darkens in the air and can be corroded by steam and acids, but resistant to alkali corrosion. Its atomic radius is 138.5pm; the ionic radii of U 3+ , U 4+ , U 5+ , and U 6+ are 103, 97, 89, and 80 pm, respectively. The electronegativity of uranium was determined by Pauling to be 1.38; Allred and Rochow were determined to be 1.22 [4] .
- Uranium can react with most non-metallic elements and their compounds. The reaction temperature and reaction speed vary with the particle size of uranium. Uranium can spontaneously ignite in air or oxygen at room temperature, and fine uranium can spontaneously ignite in water. Under certain conditions, the energy released by the oxidation of uranium can cause an explosion. The lower limit of the explosive concentration of uranium dust is 55 mg / dm 3 . Uranium can react with many metals to form intermetallic compounds. Uranium can form solid solutions with niobium, hafnium, zirconium, molybdenum, and titanium [4] .
- Uranium and its compounds are highly chemically toxic. The allowable concentration of soluble uranium compounds in the air is 0.05 mg / m 3 , the allowable concentration of insoluble uranium compounds is 0.25 mg / m 3 , and the allowable human radioactive dose to natural uranium. The uranium compound is 7400Bq and the insoluble uranium compound is 333Bq [4] .
Uranium physical properties
- Uranium is a radioactive metal element that can be used as a fuel for nuclear reactions. Uranium is a silver-white metal, almost as hard as steel, with a high density (relative density of about 18.95), a melting point of 1135 ° C and a boiling point of 4134 ° C. Before the development of nuclear energy, it was used to make yellow glass. Uranium is the element with the highest atomic number in nature. E. Paley (1811-1890) isolated metallic uranium in 1841, although uranium was previously recognized in pitch uranium mines. It is also hidden in mica uranium ore, vanadium potassium uranium ore, and monazite; it is mainly distributed in Canada, Australia, and South Africa. Various isotopes of uranium can be separated from the volatile gas uranium hexafluoride (UF 6 ) by gas diffusion technology [5] .
- There are three allotrope of uranium, their temperature and main structural characteristics are listed in the table. The density of -U at room temperature was 19.02 t / m 3 . -U and -U have obvious anisotropy. For example, between 298 and 523 K, the thermal expansion coefficients of -U single crystals along the a, b, and c axes are a = + 33.24 × 10 -6 / K , b = -6.49 × 10 -6 / K, c = + 30.36 × 10 -6 / K. -U has an isotropic structure. The thermal expansion coefficient of the randomly arranged polycrystalline uranium in the range of 293 to 373 K is equal to 16.3 × 10 -6 / K. The specific heat between 5 and 350K is 27.66J / (mol · K). The thermal conductivity of -U increases with increasing temperature, 25.1W / (m · K) at room temperature, and 37.7W / (m · K) at 1033K [3] .
- The mechanical properties of uranium vary with the sample furnace number and heat treatment. For -rolled and -annealed samples, the maximum yield strength at room temperature is 206.8-275.8 MPa. For small deformations of extruded uranium, the room temperature tensile strength limit is 586.1 to 861.8 MPa [3] . Uranium has three lattice structures: -U is an orthorhombic structure, a = 284.785pm, b = 585.801pm, c = 494.553pm; -U is a square structure, a = 1076.0pm, c = 565.2pm; -U For body-centered cubic structure, a = 352.4pm. Their transition temperatures are 941K ( ) and 1047K ( ) [4] .
| Allotrope of uranium | -U | -U |
| |
| Existing temperature range (K) | <941 | 941-1048 | 1048 (melting point) | |
| Crystal structure | Oblique | Quartet | Body-centered cubic | |
| Number of atoms in the unit cell | 4 | 30 | 2 | |
| Lattice constant (nm) | a 0 | 0. 28541 | 1.0579 | 0. 3524 |
| b 0 | 0. 58692 | - | - | |
| c 0 | 0. 49563 | 0. 5656 | - | |
- The important physical properties of uranium are listed in the following table [4] .
| nature | data |
| Melting point T / K Boiling point T / K Heat of fusion Q / kJ · mol -1 Gasification heat Q / kJ · mol -1 Density / kg · m -3 Thermal conductivity / W · m-1 · K -1 Resistivity / · m Molar volume Vm / cm 3 Linear expansion coefficient l / K -1 | 1405.5 4018 15.5 417.1 18950 (293K), 17907 (melting point liquid) 27.6 (300K) 30.8 × 10 -8 (273K) 12.56 12.6 × 10 -6 |
Uranium isotope and its half-life
- Natural uranium contains three isotopes: 238 U, 235 U, and 234 U. Their contents are 99.28% ( 238 U), 0.71% ( 235 U), and 0.006% ( 234 U), and their half lives are ( 238 U) and 4.51. × 10 9 , ( 235 U) 7.09 × 10 8 and ( 234 U) 2.35 × 10 5 years. Among them, 235 U is the most important, which is currently the fuel of nuclear power. A 235 U nucleus emits about 2.5 neutrons when it absorbs a thermal neutron for fission and releases 207 MeV energy. The energy released by 1 kg 235 U nuclear fission is equivalent to the energy produced by burning 2700t coal [6-7] . Depending on the reactor type and its operating conditions, nuclear fuel can be natural uranium or enriched uranium with increased U content. The separation of uranium isotopes by gas diffusion method, centrifugation method or laser method can make the enrichment of U reach more than 90%. U captures neutrons and transforms them into fissile Pu (). Pu and U are also the main raw materials for making nuclear weapons [3] .
- In the 25km crust, there is 10 14 t of uranium, of which 10 10 t is seawater, and an average of 3.3mg of uranium per ton of seawater. Hundreds of uranium-containing minerals exist in nature, but most of them are depleted, so economically large-scale mining is difficult. At present, uranium ore with economic value is about 0.1% of U 3 O 8 content. If fast neutron breeder reactors are developed, the utilization of uranium resources can be increased by 60 to 70 times compared to pressurized water reactors [3] .
- The most abundant uranium isotope is 238 U, and the most abundant is 235 U, which can be used as fuel for nuclear power generation. The least abundant is 234 U. There are also 12 artificial isotopes.
| isotope | Abundance | half life | Decay mode | Decay Energy (MeV) | Decay products |
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Uranium nuclear properties
- The thermal neutron absorption cross section of uranium is 7.60b ± 0.07b. There are 15 isotopes of uranium (including isonuclear isotopes), with masses ranging from 227 to 240. The natural isotopic composition of uranium is listed in the following table [4] .
| Nuclide | relative atomic mass | Natural abundance /% | Half-life T1 / 2 / a | Decay mode and decay energy / MeV |
| 234 U 235 U 238 U | 234.0409 235.0439 238.0508 | 0.005 0.720 99.275 | 2.47 × 10 5 7.00 × 10 8 4.51 × 10 9 | (4.856); (4.681); SF; (4.268); SF; |
- 235 U is the progenitor nuclide of the plutonium uranium decay system, 238 U is the progenitor nuclide of the uranium and radium system, and 234 U is the decay product of 238 U. 235 U is the only natural fissile nuclide. The 235 U nuclides are bombarded by thermal neutrons and undergo fission (induced fission) after absorbing a neutron. A 235 U nucleus emits a total energy of 195 MeV during fission, and simultaneously emits 2 to 3 (average 2.5) neutrons. As long as one of the neutrons causes fission of the other 235 U nucleus, chain nuclear fission will continue. 238 U is not a fission nuclide, but 238 U generates 239 U after neutron absorption in the active area of the reactor, and 239 U undergoes two beta decays to generate fissionable Pu. Therefore, the fast neutron breeder reactor can be used to give full play to the role of 238 U and improve the utilization rate of natural uranium [4] .
A brief history of uranium discovery
- Uranium (yóu), the English name, is named after Uranus' "Uranus". In 1789, Martin Heinrich Klaproth first discovered "uranium" from an asphalt uranium mine, and named it uranium with a newly discovered planet in 1781-Uranus, and the element symbol was U. However, in 1841, Eugene-Melchior Peligot proved that the substance was uranium dioxide, and then metal uranium was prepared by reducing UCl 4 with potassium. In 1896 Antoine Henri Becquerel discovered the radioactivity of uranium. The study of uranium at that time was purely theoretical, and uranium compounds were only used for the coloring of glass and ceramics. Radium was discovered in uranium in 1898, and uranium became a by-product of mining radium. In 1938, Otto Hahn and Fritz Strassmann bombarded uranium nuclei with neutrons and found that nuclear fission released energy at the same time, which caused people to re-emphasize uranium. During and after World War II, due to the need for nuclear weapons and nuclear power, the exploration and exploitation of uranium resources was accelerated [3] .
- The United States has set up a specialized agency for this purpose. The United States dropped the first 239 Pu atomic bomb on Hiroshima, Japan in 1945, and dropped a 235 U atomic bomb on Nagasaki, Japan a few days later. The first nuclear power plant was built in the Soviet Union in 1954. Since then, the scientific research and production of uranium have received great attention from countries around the world, and the nuclear weapon manufacturing and nuclear power generation industries have developed rapidly. Since the rise of the uranium industry in China in the 1950s, China has formed a complete and considerable scale scientific research and industrial production system [4] .
Uranium compounds
- There are many compounds of uranium. The chemical formulas, existing forms and uses of the main uranium compounds are listed in the following table [4] .
| name | Chemical formula | Existence form | use |
| Uranium dioxide | UO 2 | Dark brown or black powder | Manufacture of reactor elements or production of UF 4 |
| Uranium trioxide | UO 3 | Amorphous UO 3 or -UO 3 , brown, -UO 3 orange powder, -UO 3 Bright yellow, -UO 3 red, -UO 3 brick red, -UO 3 brown | Reduction to UO 2 |
| Uranium trioxide, uranium tetrafluoride, uranium hexafluoride | U 3 O 8 UF 4 UF 6 | Olive green (sometimes dark green or black) powder emerald crystal (green salt) Near white solid at room temperature, sublimation at 309K, the most volatile uranium compound | Storage and reduction to UO 2 to produce metallic uranium or UF 6 Isotope separation, 235 U concentrated |
| Uranyl nitrate | UO 2 (NO 3 ) 2 | UO 2 (NO 3 ) 2 · 6H 2 O bright yellow crystal | Denitrification to UO 3 |
| Ammonium diuranate | (NH 4 ) · 2U 2 O 7 | Yellow precipitates (commonly known as "yellow cakes") are flaky crystals of good quality | Thermal decomposition into UO 2 or UO 3 |
| (NH 4 ) · 4UO 2 (CO 3 ) 3 | Light yellow crystal | Thermal decomposition into UO 2 |
Uranium alloy
- Uranium can form intermetallic compounds with many metals. Uranium has such defects as lively chemical properties, poor anisotropic structure and poor mechanical properties. Certain properties of uranium alloys are superior to metallic uranium, which is important in the manufacture of nuclear fuel elements. Adding appropriate amounts of other metals, such as niobium, chromium, molybdenum, or zirconium, can improve the thermal conductivity, crystal structure and metallographic structure, heat treatment characteristics, radiation stability, and corrosion resistance of uranium [4] .
- Depleted uranium bomb is a kind of high-efficiency combustion armor-piercing bomb made of depleted uranium alloy. It is made by virtue of the density of depleted uranium material (2 times of lead density), high strength (3 times of steel strength), strong penetrating power, and easy burning , Depleted uranium alloy contains 238 U, 235 U, etc. The residual 235 U after the depleted uranium bomb can damage the kidneys and nervous system and cause lung cancer. Depleted uranium alloys with high density and hardness can also be made into radiation-proof materials [8] .
Uranium Occurrence and Resources
- Uranium is widely found in the crust and sea water. The concentration of uranium in seawater is 3 × 10 -7 %, and the crustal abundance is 2.3 × 10 -4 %, but it is dispersed in the crust [4] .
Uranium Mineral
- Uranium minerals can be divided into two categories: primary uranium ore and secondary uranium ore. Except for pitch uranium ore, primary uranium ore exists in pegmatite, and the primary mineral is easily transformed into various secondary minerals through weathering and hydrothermal action. The genesis, occurrence, uranium content, and associated minerals and surrounding rocks of uranium ore will affect the processing technology of uranium ore [4] . About 500 types of uranium minerals and uranium-containing minerals have been discovered. Among them, only twenty or thirty are common and have industrial practical value. The following table lists important uranium minerals [4] .
| Types of | Mineral name | composition | Uranium content (mass fraction ) /% |
| Primary uranium ore | Asphalt uranium crystalline uranium ore
| UO 2 · mUO 3 · nPbO (U, Th) O 2 · mUO 3 · nPbO (U, Ce, Fe, Y, Th) 3 Ti 5 O 16 |
|
| Secondary uranium ore | Water (asphalt) uranium ore | UO 2mUO 3nH 2 O | |
| Copper uranium mica calcium uranium mica potassium vanadium uranium ore vanadium calcium uranium ore | Cu (UO 2 ) 2 (PO 4 ) 2 · (8 12) H 2 O Ca (UO 2 ) 2 (PO 4 ) 2 · (8 12) H 2 O K 2 (UO2) 2 (VO 4 ) 2 · (1 to 3) H 2 O Ca (UO2) 2 (VO 4 ) 2 · 8H 2 O | 50 50 50 50 60 |
- Except for some large uranium-rich deposits in Australia and Canada (containing 1% to 10% uranium), most uranium deposits have a uranium-containing grade of 0.1% to 0.2%. Most uranium hydrometallurgical plants directly process uranium ore, but through ore dressing, ore grade can be improved and costs can be reduced. Some countries are using radio-separation machines for beneficiation of massive uranium ore. In order to comprehensively utilize or improve the processing performance of uranium ore, some ore also need to be pretreated by ore blending and roasting [4] .
- In addition, uranium-containing phosphate ore, lignite, shale, uranium mineral water, uranium-containing copper waste rock leachate, and seawater can be used as raw materials for uranium extraction. For example, in 1988, the United States recovered 1,500 tons of uranium from by-products such as wet phosphoric acid, accounting for about 29% of its total production of 5,190 tons of uranium. [4]
Uranium distribution range
- Copper uranium mica
- The average content of uranium in the crust is about 2.5 parts per million, that is, about 2.5 grams of uranium per ton of crust material, which is higher than the content of tungsten, mercury, gold, silver and other elements. The content of uranium in various rocks is very uneven. For example, the content in granite is higher, with an average of 3.5 grams of uranium per ton. The first layer of the earth's crust (20 km from the surface) contains nearly 1.3 × 10 14 tons of uranium. Based on this calculation, one cubic kilometer of granite will contain about 10,000 tons of uranium. The concentration of uranium in the seawater is quite low, with an average of only 3.3mg of uranium per ton of seawater, but because the total amount of seawater is very large (the total uranium content in the seawater can reach 4.5 × 10 9 tons), and it is convenient to extract from water Therefore, many countries, especially those lacking uranium resources, are exploring ways to extract uranium from seawater [1] .
- Because the chemical nature of uranium is very active, there is no free metal uranium in nature, and it always exists in a combined state. There are more than 170 known uranium minerals, but there are only two or thirty uranium mines with industrial mining value. The most important of these are pitch uranium ore (the main component is uranium octoxide), and quality uranium (Main component is uranium dioxide), uranium stone and uranium black. Many uranium minerals are yellow, green or yellow-green. Some uranium minerals emit strong fluorescence under ultraviolet light. It is the fluorescent nature of uranium minerals (uranium compounds) that led to the discovery of radioactive phenomena [1] .
- Green uranium ore
- Although the distribution of uranium elements is quite wide, the distribution of uranium deposits is limited. Uranium resources are mainly distributed in the United States, Canada, South Africa, West South Africa, Australia and other countries and regions. It is estimated that the proven industrial reserves exceeded 1 million tons by 1972 [1] .
- The radioactivity of uranium and its series of decay progeny is the best sign of its presence. Although the human eye cannot see the radioactivity, it can be easily detected with the help of specialized instruments. Therefore, almost all uranium resource surveys and explorations take advantage of the radioactive nature of uranium: if it is found that rocks, soil, water, or even plants are particularly radioactive in a certain area, it may indicate that there is a uranium mine in that area [1] .
Uranium extraction metallurgy
- Extraction metallurgy of uranium has three characteristics. First, the grade of uranium ore is very low. Generally, it contains 0.1% to 0.2% of uranium ( 238 U + 235 U), and 235 U is only 0.0007% to 0.0014%. In order to obtain nuclear pure uranium, a series of enrichment, Purification process. Second, nuclear pure metal uranium needs to be separated by isotopes to make 235 U with different abundances. Third, the process is complicated and there are radiation hazards. Therefore, the production technology of uranium is difficult and the safety protection requirements are strict [4] .
- Except for in-situ leaching and uranium extraction from other uranium-containing materials, uranium extraction metallurgy begins with uranium ore (raw or concentrate), and usually includes uranium extraction, uranium tetrafluoride, metal uranium, six The main steps of uranium fluoride production, uranium isotope separation, preparation of enriched U-containing metallic uranium and processing of uranium fuel elements [4] .
- (1) Uranium extraction. Including leaching of uranium ore, liquid-solid separation, enrichment, purification (commonly used ion exchange or solvent extraction methods) and thermal decomposition of precipitated products to produce nuclear pure UO 2 or U 3 O 8 [4] .
- (2) Preparation of uranium tetrafluoride. UO 2 (U 3 O 8 can be reduced to UO 2 with hydrogen) to hydrofluorinate to uranium tetrafluoride (green salt) [4] .
- (3) Preparation of metallic uranium. Reduction of UF 4 to metallic uranium with metallic calcium or magnesium, which is then refined, cast, processed, forged (or extruded) formed, and cladding to make reactor elements for natural uranium for the production of fission element 239 Pu [4] .
- (4) Preparation of uranium hexafluoride. UF 4 is fluorinated to UF 6 [4] .
- (5) Uranium isotope separation. Utilizing the slight difference between the masses of 235 U and 238 U, the concentrated 235 UF 6 with different abundances was prepared by gas diffusion (or centrifugation) of UF 6 [4] .
- (6) Preparation of metallic uranium containing enriched 235 U. The concentrated 235 UF 6 is reduced to 235 UF 4 by hydrogen, and then converted into concentrated 235 UO 2 . Reduction of 235 UF 4 with calcium or magnesium produces metallic uranium containing concentrated U [4] .
- (7) Processing of uranium fuel elements. The enriched UO 2 or metallic uranium is further processed into reactor fuel elements or other end products [4] .
Uranium safety protection
- Uranium and its compounds emit both harmful radiation and chemical toxicity. Safety measures must be taken during production. The main contents of safety protection measures include strictly preventing dust and radon from entering the human body; making the radiation dose at the production site lower than the limit set by radioactive health protection; the discharged three wastes must meet the nationally prescribed emission standards after treatment. Attention should be paid to the critical safety of enriched uranium products [4] .
Development Trend of Uranium
- Utilize low-grade uranium ore and other uranium-containing materials to expand uranium resources. Attach importance to the development and promotion of leaching methods such as in-situ leaching, heap leaching, and bacterial leaching to save energy and reduce production costs. Develop and promote the application of fast neutron breeder reactors to improve the utilization of natural uranium. Research and develop waste-free processes to reduce uranium's environmental pollution. Develop energy-efficient centrifugal and laser methods equivalent to isotope separation methods, replacing the energy-intensive diffusion method [4] .
Uranium uranium nuclear fission
- In nature, 234 U does not undergo nuclear fission. Generally, 238 U does not undergo fission. Only 235 U is prone to nuclear fission. Nuclear fuel mainly refers to 235 U. The half-life of 235 U is 7.038 × 10 8 years. Starting from 235 U, after 11 consecutive decays, a stable 207 Pb finally appears. The half-life of 238 U is 4.468 × 10 9 years. Starting from 238 U, after 14 consecutive decays, the stable 206 Pb-206 finally appears. In the continuous decay of 238 U, the longest half-life of the nucleus is 234 U, and its half-life is 2.45 × 10 5 years [8] .
- 235 U, 233 U, and 239 Pu are the main nuclear fissionable materials, which can be directly used as nuclear fuel. They can be obtained in large quantities and easily absorb slow neutrons (energy less than 1eV) and fission. 234 U and 238 U are not. 235 U exists in natural uranium, and 233 U and 239 Pu are produced by uranium nuclear reactors. For 235 U, 233 U, and 239 Pu, neutrons of any energy can split them and release energy; for 235 U, the slower the neutron, the more likely it is to cause fission. 238 U absorbs a neutron and can also be transformed into fissionable matter [8] .
- Both 235 U and 238 U can spontaneously fission, but the probability of spontaneous fission is very small [8] .
235U Uranium 235U fission
- Studies have shown that after 235 U absorbs slow neutrons, there are more than 40 fission modes, which can produce at least 36 kinds of elements of more than 300 nuclides and fast neutrons (average 2.5), and release huge energy. In addition to neutrons, uranium nuclear fission products usually have two (two-split) fission products, and three (three-split) and four fission products (in 1946, Chinese physicist Qian Sanqiang and others found in France) "Triple splits" are extremely unlikely. In addition to neutrons, there are many ways to combine the "two splits" of uranium nuclei. The ratio of the masses of fragments is roughly 3: 2, and the chance of the same mass is the smallest. It is an alpha particle, and the probability of occurrence of the "triple split" is 3/1000 of the "second split". Statistics show that the neutron energy (kinetic energy) emitted by 235 U fission is in the range of 0.1-20 MeV, with an average of 2 MeV. Fast neutrons alone cannot produce a continuous fission chain reaction of natural uranium, nor can slow neutrons cause fission of 238 U. Continuous fission reactions cannot occur in 238 U. 235 U and 240 Pu, etc. In addition to neutrons that can trigger their nuclear fission, charged particles or gamma rays with sufficient energy can also trigger fission. In addition, uranium also generates capture resonances for neutrons of about 25 eV, that is, capture without fission [8] .
- 235 U has a small binding energy, a low nuclear fission barrier, and neutrons of any energy can make it fission, and it has a slow neutron (neutron rate of 2.2 × 10 3 m / s, which is comparable to the gas molecule motion rate at room temperature In this way, it stays relatively near the uranium nucleus for a long time, and it is easy to hit the uranium nucleus and cause it to fission. It has a large fission cross section (the probability of fission is large). 235 U absorbs a slow neutron. Usually, the excited state of 236 U (re-nucleation) is formed, then it is split into two pieces, and neutrons and energy are released at the same time. In a thermal neutron (a kind of slow neutron) reactor, the thermal neutron fission cross section of 235 U is 200 times larger than the thermal neutron fission cross section of 238 U. In this way, there will be a sufficient number of neutrons to cause 235 U nuclear fission. This can make up for the weakness of less 235 U in natural uranium or enriched uranium; the utilization rate of uranium is 1% -2% when this kind of reactor is working [8] .
238U Uranium 238U fission
- 238 U ( 240 Pu, 232 Th) fission is valved, and neutrons less than 1.1 MeV will be absorbed or scattered by them, and can not cause fission; neutrons with larger energy can make them fission, but the possibility is very small. 238 U has a large binding energy and a high fission barrier. Fast neutrons with an energy exceeding 1.4 MeV can make it fission and release large neutron energy. Studies have shown that 238 U has many resonance absorption peaks above a few MeV, and its fission probability increases with increasing neutron energy. 238 U is not prone to fission, but it can become better nuclear fission materials such as 239 Pu and 233 U after neutron absorption. The probability of thermal neutrons being captured by 238 U is about 1/190 of the probability that thermal neutrons will cause fission of 235 U. The main role of fast neutrons and 238 U nuclei is inelastic collisions. Most neutrons reduce energy through inelastic collisions and are absorbed by 238 U nuclei in multiple collisions [8] .
Uranium enrichment
Brief introduction of uranium enrichment
- Uranium material contains more than 0.7% of 235 U, which means enriched uranium. Uranium is an important nuclear fuel and nuclear bomb core material. All uranium materials used today need to be refined and concentrated to achieve a certain purity. For example, the production of an atomic bomb requires at least 20-50 kg of highly enriched uranium (the atomic bomb can also be made of plutonium), and its enrichment purity should reach more than 90% [8] .
- The content of 235 U in uranium ore is very low, and most of it is 238 U. Uranium ore cannot be directly used as fuel for coal blocks. It is similar to most coal cakes that are mud and cannot be burned. The abundance of the uranium element isotope (the ratio of the number of atoms of a certain isotope to the total number of atoms in an isotope mixture of an element) can be increased by processing to obtain a certain isotope enriched by the element, such as 235 U, The physical properties of 235 U and 238 U are slightly different, mainly due to the difference in relative mass, which results in a slight difference in the quality of the compound particles they form. The separation of uranium natural isotopes with different atomic weights can make the ratio of 235 U to 238 U in uranium material higher than that of natural uranium, thereby obtaining fissile material-enriched uranium. "Enrichment" mainly refers to an isotope separation process that increases the abundance of a specific isotope of an element, such as the production of enriched uranium from natural uranium or the production of heavy water from ordinary water.
- The purpose of enriching uranium isotopes is to increase the relative abundance (concentration) of 235 U compared to 238 U, so that the relative content of natural 235 U is higher than 0.7% of uranium, that is, enriched uranium, and the content of 235 U in uranium fuel reaches more than 3%. It is possible to continue to "burn"; enriched uranium includes: 3%, 3.5%, 20% enriched uranium and other species. The uranium used in many nuclear reactors and weapons must be enriched, that is, the concentration of 235 U, which is prone to nuclear fission, must be increased and then made into fuel. The uranium fuel 235 U concentration in most nuclear power plants' nuclear reactors is about 3% and no more than 5%. The concentration of 235 U in uranium materials in nuclear weapons needs to be above 90%; the uranium fuel used by nuclear ships is 20% or lower. The International Atomic Energy Agency defines: 235 U uranium materials with an abundance of 3% are low-enriched uranium (industrial-grade nuclear fuel) for nuclear power plants, often uranium salts or uranium oxide; uranium materials with an abundance greater than 80% are highly enriched uranium, with an abundance More than 90% are weapon-grade (military) highly enriched uranium, which is mainly used for manufacturing nuclear weapons; the other division is: highly enriched uranium (abundance above 20%), and low enriched uranium (2% -20%, commercial Enriched uranium), slightly enriched uranium (0.9% -2%), and weapon-grade enriched uranium (over 90%). The uranium enrichment concentration of 20% is a node and a hurdle. Therefore, it is a relatively easy process to increase the uranium enrichment [8] .
- Obtaining uranium material requires a series of complex processes, including processes of prospecting, mining, beneficiation, leaching, smelting, refining, etc. Separation and enrichment are the main and difficult processes, with high technological content. The relatively pure uranium ore product obtained after milling and sorting of uranium ore, that is, uranium concentrate, also called "yellow cake", the main components are uranium trioxide, sodium diuranate, and ammonium diuranate. It is a nuclear fuel An intermediate product in the production process. Generally, more than 200 tons of higher grade uranium ore are needed to obtain 1 kg of weapon-grade 235 U [8] .
- Whether it is the peaceful use of nuclear energy or the manufacture of nuclear weapons, enriching uranium is necessary. As of November 2006, most of the 470 commercial nuclear power reactors operating or under construction in the world were enriched uranium as fuel; by 2010, there were at least 1,600 tons of highly enriched uranium (and 500 tons of plutonium) worldwide, and China is the world The fourth country (the United States, Britain, and the Soviet Union) to independently control the production technology of uranium enrichment; in the early 1960s, China established the Hengyang Uranium Water Smelting Plant and the Lanzhou Gas Diffusion Plant to obtain enriched uranium (5 May 1958). In January, Lanzhou started to build China's first uranium enrichment production enterprise, which provided high-quality nuclear fuel for China's first atomic bomb, first hydrogen bomb, first nuclear submarine, and first nuclear power station). In 2011, a research report from the Belfort Center at Harvard's Kennedy College stated that China has 16 tons of military-grade uranium and 1.8 tons of plutonium. [8]
Uranium enrichment technology
- Due to the issue of nuclear weapons, uranium enrichment technology has always been a sensitive technology forbidden by the international community. The IAEA and the United Nations hope to control uranium enrichment activities in various countries. Taiwan, China had a nuclear program. Since 1985, Romania has been secretly engaged in the refining of weapons-grade uranium; in 1991, the Romanian government also placed its nuclear equipment and research under the supervision of the International Atomic Energy Agency; in 2003, the United States and Russia placed more than a dozen kilograms, Up to 80% of the enriched uranium is shipped to Russia for processing. Libya violated international commitments and secretly acquired uranium enrichment technology to develop nuclear weapons. After the "Iraq War" in 2003, the country compromised with the West and handed over the relevant equipment and drawings to the United States and Britain [8] .
- In addition to several nuclear universities, Japan, Germany, India, Israel, Pakistan, Argentina, North Korea (In January 2011, the UN Security Council's "Sanctions Committee against North Korea" expert group drafted the "North Korean Uranium Enrichment Plan Report" with a view to preventing nuclear proliferation , Nuclear contest upgrade), Iran (June 12, 2011, Iran s Permanent Representative to the International Atomic Energy Agency, Sultaniye, told Xinhua News Agency reporters during the Second International Conference on Nuclear Disarmament that Iran has produced more than 50 kg with a purity of 20 % Of enriched uranium, Iran's goal is to have 120 kg of this nuclear material) and other countries have mastered uranium enrichment technology [8] .
Uranium enrichment method
- The technology for purifying and concentrating 235 U is more complicated. Various isotopes of uranium elements, like "twin sisters", have very similar physical and chemical properties. Utilizing small differences in physical and chemical properties of isotopes, the ratio of uranium element isotopes can be changed through methods and processes such as diffusion, evaporation, or chemical exchange. Treatment of natural uranium by the isotope separation method can increase the concentration of 235 U and make the relative content of 235 U in natural uranium higher than 0.7%, thereby obtaining uranium materials with different concentrations for various needs. Technologies for separating uranium isotopes on industrial scale (applicable to increase U-235 concentration) include gas diffusion method, gas centrifugation method, ion exchange method and distillation method, electrolytic method, galvanic method, liquid thermal diffusion method, electromagnetic separation method and laser separation method Wait. These enrichment methods have complicated processes, large investment, high energy consumption, and low output, that is, the cost of producing uranium fuel is large [8] .
Uranium gas diffusion method
- This is the earliest and most mature enrichment method, and it is also the first enrichment method commercially developed. It is based on the principle of molecular permeation and diffusion, and uses different uranium isotopes of different masses to separate at different rates of movement. UF 6 is a highly toxic, corrosive, and radioactive white crystal, which heats up into a gas. Since the masses of 238 U and 235 U are different, the molecular masses of the two in UF 6 gas are also different. The mass of 235 U in UF 6 is 349 and the mass of 238 U is 352. When a high-pressure UF 6 mixed gas (a mixture of uranium isotopes) passes through a porous membrane installed in a cascade, U-235 light molecular gas in UF 6 will pass through the porous membrane faster than 238 U heavy molecular gas in UF 6 . The gas passing through the membrane tube is immediately pumped to the next stage, and the gas remaining in the membrane tube is returned to the lower stage for recirculation. In each gas diffusion stage, the concentration ratio of 235 U to 238 U only slightly increased, so separation and concentration to the industrial-grade 235 U concentration required more than 1000 levels [8] .
- The core of this technology is a porous diffusion separation membrane (the Soviet Union once called it the "heart of the socialist camp safety"). In 1964, China developed a high-quality separation membrane element (called a separation membrane at the time. Invention First Prize). Separation membranes are porous thin metal plates or films with millions of ultra-fine pores per square decimeter. These films (plates) are rolled into tubes and packed in sealed diffusers. In the cascade device composed of these tubes, the mixed gas will be gradually separated. The concentrated UF 6 gas containing more than 235 U flows forward along the cascade device, and the thin UF 6 gas containing more than 238 U falls due to the flow lag. Diversion. The diffusion and concentration process of this method requires thousands of continuous cascade devices. Continuous diffusion can separate molecules containing 238 U from molecules containing 235 U in the UF 6 mixed gas, and then chemically treat the concentrated UF 6-235 U gas molecules, which in turn yields 235 U. This method of uranium enrichment has low efficiency and high energy consumption [8] .
Uranium gas centrifugation
- In 1919, the German scientist G. Ruipi completed the basic design of the gas centrifuge. The concept and application of uranium enrichment centrifuge was developed by J. W. Proposed by Bermos. In 1934, Bermos successfully separated two chlorine isotopes; in 1941, he and his colleagues successfully separated uranium isotopes for the first time using a centrifuge. Separately, three centrifuges were designed. Their work attracted the attention of the leaders of the Manhattan Project (Plan for Atomic Bomb Development) that was being implemented in the United States at the time, but the United States ultimately chose the gas diffusion method [8] .
- The gas centrifugation method is also suitable for processing uranium mixed liquid or uranium vapor. It uses a uniquely designed centrifuge to make the gas or liquid flow in each centrifuge without interruption, and can continuously operate to process uranium gas stream or uranium liquid stream. Currently, this mechanical separation method is commonly used for enriched uranium. In this method, a vacuum high-speed centrifuge is a key device, and the presence of this device is often used internationally as a sign to determine whether a country is conducting nuclear weapons research. Compared with the gas diffusion method, the gas centrifugation method has higher ergonomics and requires significantly less electricity, so this method has been adopted by most enriched uranium plants [8] .
- High-pressure UF 6 gas is injected into a high-speed closed centrifuge. Due to the difference in mass, long-term rotation depends on inertial centrifugal force. The 235 U molecules in the lighter UF 6 are mostly concentrated at the shaft of the container, and the heavier UF 6-238. U molecules mostly cluster at the edges.UF 6 - 238 UUF 6 - 235 UUF 6 - 235 UUF 6 - 235 U235 UUF 6 - 235 U[8]
- 20101/5201123600 [8]
- 1
- [8]
- 235 U1/21/1019771982238 U 235 U[8]
- 1234 U 235 U 238 U235 U235 U238 U235 U235 U[8]
- 2UF 6 16UF 6235 U 235 UUF 6235 U235 U235 U235 U2000[8]
- UF 6 [8]
- 2
- UF 6235 UUF 6UF 6UF 6UF 6[8]
- 3
- 235 U235 U20 402080[8]
- 4
- 1 196410235 U195162.5 3[8]
- 1942235 U1kg 235 U1800tTNT[4]
- 501kg 235 U2700t[4]
- [4]
- Scientific research and industrial practice have proven that uranium is the only natural nuclear fuel, and the nuclear energy industry must rely on uranium. Because the nuclear energy industry has both peaceful and military applications, uranium has become a special commodity metal, and its production is affected by many political, social, and economic factors. In the 1940s and 1950s, uranium was mainly used for nuclear weapons, and after the 1950s it was mainly used for nuclear power generation. The world's uranium production has been oversupply for a long time, and there are large stocks. The price of U 3 O 8 per kilogram in the international market decreased from US $ 97 in early 1978 to US $ 19.84 in 1990. The annual output of uranium in western countries also decreased from 43960t in 1980 to 35278t in 1985. But during this period, nuclear power plants developed rapidly, with a total installed capacity of 135 million kW in 1980 and an increase to 318 million kW in 1989. The annual production of uranium in 1985 was lower than the demand for nuclear power generation [4] .
Uranium atom bomb
- Regularly place conventional explosives around uranium, and then use electronic detonators to make these explosives accurate
- Atomic Bomb Mushroom Cloud