The uranium enrichment is the process consisting in increasing the proportion of isotope Fissile in the Uranium. The most common operation is natural uranium enrichment in its isotope 235. By extension, enrichment is also the content of fissile material.

Natural uranium contains 0,71  % of Uranium 235. However to cause a reaction of Nuclear fission in the pressurized water reactors, it is necessary to have a uranium which contains between 3 and 5  % of the Isotope 235. Two isotopes 235 U and 238 U having the same chemical properties, one bases oneself on a physical property, the light difference of molecular Mass. Uranium enrichment is at the base of the reactor families generating to ordinary water (REFERENCE MARK and REB). This technology also founds the development of the atomic weapons with enriched uranium.

Classification of uranium according to its enrichment

Natural uranium

Natural uranium (in English natural uranium - NAKED) has a content Uranium 235 of 0,7%.

Slightly enriched uranium

Slightly enriched uranium (in English, Slightly enriched uranium - SEU) has a concentration of ²³⁵U ranging between 0.9% and 2%. + From the point of view of the international regulation, the uranium whose enrichment lies between 0.7% and 20% known as is slightly enriched. It is in particular the case of the uranium intended for nuclear fuel of the generating power stations. This quality of enrichment is intended to replace natural uranium like fuel in certain types of Heavy-water reactor, like the Réacteur CANDU. A light enrichment makes it possible to optimize the costs, because the loading consequently requires less uranium. This reduces the quantity of consumed fuel, and the later cost of management of waste. + the uranium of reprocessing (URT) is resulting from the treatment irradiated fuels UOX. Its enrichment is about 1% and its isotopic composition is more complex (notable presence of uranium 234). Uranium D-nouveau riche (URE) is uranium of reprocessing which underwent a new stage of enrichment, bringing it to a content Uranium 235 of some pourcents. Reprocessed uranium (in English, Recovered uranium - RU) is a type of slightly enriched uranium. It is produced in the cycles of light Water reactor: the worn nuclear fuel of these dies contains with final proportion of U-235 higher than the natural content, and can be used in the engines which consume natural uranium or slightly enriched.

Slightly enriched uranium

From the point of view of the international regulation, the uranium whose enrichment in ²³⁵U remains lower than 20% known as is slightly enriched. It is in particular the case of the uranium intended for nuclear fuel of the generating power stations. Slightly enriched uranium (in English, Low-enriched uranium - LEU) is typically used atrates of enrichment from 3 to 5% in light Water reactor, the type of engine more running in the world. Engines of research use rates of going enrichment from 12% to 19.75%, this last concentration in extreme cases lawful being used like substitute product to make function engines initially designed for uranium highly enriched.

The uranium of reprocessing (URT) is resulting from the treatment irradiated fuels UOX. Its enrichment is about 1% and its isotopic composition is more complex (notable presence of uranium 234). Uranium D-nouveau riche (URE) is uranium of reprocessing which underwent a new stage of enrichment, bringing it to a content Uranium 235 of some pourcents.

Uranium highly enriched

Beyond of a concentration in ²³⁵U higher than 20%, uranium is regarded as " highly enrichi" (in English, Highly enriched uranium - HEU) by international conventions. This quality of uranium is used in certain types of Fast reactor, and the nuclear marine propulsion, rates which can be ranging between 50% and 90%. The civil engine Fermi-1 functions thus ata rate of nominal enrichment of 26.5%.

The world uranium stock highly enriched was about 2_000 tons in 2000 (to be compared with the 2_300_000 tons uranium produced on the whole in the world).

20%, the content of isotope 235 (or 233) necessary in practice for military applications exceed 85%. For rates of enrichment of about 90%, uranium highly enriched is known as of military quality. It is usable to manufacture a Nuclear weapon. The critical mass necessary for a uranium enriched to 85% is about 50 kilograms. It is possible to manufacture atomic bombs with lower rates of enrichment, up to 20% (even less, for certain authors), but this possibility is rather theoretical: the critical mass necessary is all the more large as the rate of enrichment is weaker. When the rate of enrichment is weaker, the d'²³⁸U presence inhibits the chain reaction, which is added to the effect of dilution of the ²³⁵U. It is theoretically possible to decrease the critical mass necessary with reflectors for neutron, and/or by making imploser the load, but these techniques are in practice accessible only to countries which have already sufficient experience in the design of atomic weapons.

Depleted uranium

The residue of enrichment is depleted uranium whose U 235 content is about 0,2 to 0,3%. The depleted uranium is used in the manufacture of the Combustible MOX for the REFERENCE MARKS or the RNR and has other marginal uses (shell, ballasts…). The majority of the production is stored because it contains uranium 238 (for more than 99,3%), fertile Isotope likely to be employed in dies with Surgénérateur.

Processes of enrichment

There exist several methods of enrichment.

Historical processes

; Thermal diffusion process The thermal Diffusion uses the transfer of heat through a fine layer of liquid or gas to obtain an isotopic separation. The process is based on the fact that gas the molecules comprising of the atoms of ²³⁵U, a little lighter, tend to diffuse towards hotter surfaces, whereas the molecules containing ²³⁸U, comparatively heavier, tend to diffuse towards a cold surface.

This process was employed historically with the S-50 factory in Oak Ridge (Tennessee, the USA), during the second world war, like first stage of enrichment before an electromagnetic isotipic separation. The process was since abandoned with the profit of the gaseuse diffusion.

; Electromagnetic separation process In the electromagnetic process of separation process (in English - EMITTED), metal uranium is vaporized, then ionized. The cations thus produced are accelerated, then deviated by a magnetic field. The difference in mass between isotopes 235 and 238 creates a difference in the report/ratio of the electric charge on the mass. The flow of ions is accelerated by an electric field and is deviated by a magnetic field. The difference on the e/m report/ratio leads to a différencielle deviation which makes it possible to enrich uranium.

See also: Spectrometry mass

Historically, uranium was enriched by electromagnetic separation process during the Projet Manhattan. A mass spectrometer of industrial size was built, the Calutron; it was him which produced the matter necessary for the atomic bomb Little Boy released above Hiroshima. This method, rather ineffective, was largely since abandoned.

Gas diffusion

The gas diffusion : this process is based on the difference in mass, very low, existing between the molecules of Uranium hexafluoride 235, lighter than those of uranium hexafluoride 238. While making them filter through adapted membranes, one arrives while multiplying the number of cycles sufficiently to obtain enriched uranium.

The gas diffusion requires approximately 60 times more energy than the process of ultracentrifugation, is 6% of the energy which will be finally produced with the Uranium enriched resulting.

It is a method used since the cold war, which tends today to being replaced by less expensive processes.

Aerodynamic enrichments

; Ultracentrifugation This process is based him also on the difference in mass between the uranium hexafluoride molecules (mixture of 238 UF and 235 UF), this process consists in using very Centrifugeuse S turning at high speed. Heavier uranium 238 is found projected gradually in periphery.

; Enrichment by laminar flow Enrichment by separation of flow uses the centrifugal force created by the passage of the gas jet in a fixed vortex. The principle of separation (the gradient of pressure due to the differences in masses molecular) is the same one as in the ultracentrifugation, the advantage of the device being of eliminating the mobile machine elements. The effect is improved by diffusing the uranium hexafluoride in hydrogen or helium, which improves the speed of flow without slowing down the diffusion of UF6 in the gas vein. This process was developed in South Africa, and an experimental factory was built in Brazil. However, it is not employed on industrial scale, because of its great energy consumption.

; Zippe centrifugal machine The Zippe centrifugal machine is an alternative of the standard centrifugal machine, where the bottom of the cylinder in rotation is heated, which created currents of convections which tend to involve U235 upwards, where it is collected. This technology was employed in Pakistan, and perhaps in North Korea.

Chemical separation

The process Chemex : a chemical process, which consists of exchanges repeated between an aqueous trivalent uranium phase and an organic tetravalent uranium phase. Enrichment is based on the idea that the speed of chemical reaction is not rigorously the same one for the two isotopes. The solutions are prepared by using natural uranium and not UF₆. This technology did not become a sufficient ripe to be developed.

Separation by laser

The Isotopic Séparation by Laser on the Atomic Vapor of uranium (SILVA) exploits the light difference of electromagnetic Specter between U-235 and U-238, due so that the environment of the external electrons is not completely the same one in the two atoms. By adjusting suitably the frequency of the light exitatrice, it is then possible to excite one selectively or the other isotope, until tearing off the external electron and ionizing it.

In this technique, uranium metal is vaporized, and of the beams laser light this vapor and selectively ionize the Uranium 235, which is collected on negatively charged plates. Uranium 238, still neutral, condense on the roof of the separator. This technology did not become a sufficient ripe to be developed.

Industrial applications

In 2001, the world capacities of enrichment rose to 50 million [[UTS]], distributed per annum appreciably with parity between two technologies.

The technology of gas diffusion equips the French factory with Tricastin - Georges Besse (10.8 million UTS/an) as well as the American factory with Paducah (11.3 million UTS/an) and the Chinese factory with Lanzhou (0.45 million UTS/an). Operation with full capacity of the factory Georges Besse uses the power of three of the four engines of the site of Tricastin.

The technology of ultracentrifugation is employed in Germany (Gronau : 1.3 million UTS/an), in Japan (Rokkasho-Mura : 1.05 million UTS/an), in the Netherlands (Almelo : 1.5 million UTS/an), in the Federation of Russia (4 factories for a total of 20 million UTS/an), in the United Kingdom (Capenhurst : 2 million UTS/an) like in China (Shaanxi : 0.45 million UTS/an) which has two technologies. The Russian factory of Seversk to the characteristic to be able D-to enrich uranium by reprocessing. This technology is also at the base of the project of the factory Georges Besse II which must replace the first factory of enrichment of Eurodif to its closing, envisaged in 2020.

Notes and references of the article

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