Why is depleted uranium called depleted uranium
The Uranium enrichment is the most important large-scale isotope separation. It is used to produce nuclear fuel for nuclear reactors and for nuclear weapons.
Uranium is the only heavy element for which isotope separation is carried out on an industrial scale. In a uranium enrichment plant, the natural uranium fed in (“feed”) is separated into two fractions, one of which (“product”) has a higher proportion than the starting material and the other (“tails”) has a lower proportion 235U owns. Natural uranium consists of around 99.3% 238U and 0.7% off 235U. Nuclear reactors of the most common types (pressurized water and boiling water reactors) are mostly charged with uranium 235U content has been increased to 3 to 5%. Natural uranium can also be used in heavy water and graphite-moderated reactors. Very high enrichment is required for nuclear weapons (typically at least 90%); this uranium mixture is also known as HEU (Highly Enriched Uranium). The work performed by a separation device is expressed in kg uranium separation work (kg UTA) or tons of uranium separation work (t UTA). In the English specialist literature, the unit SWU (Separation Work Unit) is used instead of kg UTA. A large plant has an annual capacity in the order of a few 1000 t UTA. The common industrial processes use uranium hexafluoride (UF6), the only chemical compound of uranium that has sufficient volatility at room temperature (about 100 mbar vapor pressure at room temperature).
The by-product of the enrichment is depleted uranium with a 235U content of approx. 0.3%. In the case of enrichment for civil purposes, around 5.5 tons of depleted uranium are produced per ton of nuclear fuel. The depleted uranium is used, among other things, because of its high density in counterweights for aircraft wings and racing yachts, for neutron shielding and militarily in uranium ammunition. So far, only about 5% of the accumulated depleted uranium has been used for such purposes, the rest is stored. The main interest in this material, especially from Russia, is its use as a mixed material ("blender material") for the conversion of highly enriched (military) uranium into low-enriched (civil) uranium for use in light water reactors. The disarmament campaign under the START II agreement should be mentioned here in particular: “Megatons to megawatts”. According to the Atomic Energy Act of the Federal Republic of Germany, depleted uranium is a valuable material.
Diffusion plants still have the largest share of the total accumulation capacity installed worldwide (see below). However, the production share of centrifuge systems is increasing due to the technical dominance of the advanced gas centrifuges. In France, the existing gas diffusion system is soon to be replaced by a modern centrifuge system (Georges Besse II). Two new centrifuge plants are planned in the USA.
The laser enrichment, in the development of which considerable resources were invested, could not meet the expectations placed in it. Most countries have now withdrawn from this technology or at least significantly reduced the research effort.
The following table gives an overview of the most important existing plants (with capacities over 100 t UTA / a):
|country||investment||operator||Procedure||t UTA / year|
|Russia||Rostov on Don||Techsnabexport||centrifuge||1.400|
|United States||Paducah||USEC||Gas diffusion||11.300|
In the Gas diffusion method if gaseous uranium is left in the form of uranium hexafluoride (UF6) diffuse through a porous membrane. The driving force here is the pressure difference on both sides of the membrane. Molecules that 235U included are lighter than that 238U-containing and diffuse faster. In the case of a uranium isotope mixture, the gas flow that diffuses through the pores in the wall ("product") therefore contains a slightly higher proportion of the isotope U-235 than the original flow ("feed"). A single separation stage has a low separation factor (concentration ratio of the U-235 in Product and Tails) of a maximum of 1.004, but a high material throughput. For a degree of enrichment that is sufficient for the operation of light water reactors, around 1200 stages connected in series are required, which together form a so-called "cascade". The energy consumption is high and amounts to around 2300 - 2500 kWh per kg uranium separation work (UTA).
Instead of the pressure difference, a temperature gradient can also be used to separate isotopes by means of diffusion. In the thermal diffusion method (Thermal diffusion) a gas or a liquid in a narrow space between two vertical plates is heated by one of these plates and cooled by the other. Molecules that contain the lighter isotope diffuse preferentially to the warmer plate, the others to the colder plate. In addition, a slight upward convection forms on the warmer plate, so that the molecules with the lighter isotopes are concentrated in the upper area of the cell and the heavier ones in the lower area. In practice, instead of plates, concentric tubes are used (separation tube according to Clusius and Dickel).
See also:Gas diffusion process
Enrichment by gas centrifuges
The gas centrifuge process is now the more common process for uranium enrichment in the international area and has meanwhile overtaken gas diffusion in terms of importance. The most important reasons for this are the considerably lower energy consumption (around 50 kWh of separation work per kg of UTA. For comparison: diffusion separation of up to 2500 kWh of separation work per kg of UTA) and the greater flexibility in terms of capacity planning.
In the gas centrifuge process, gaseous uranium hexafluoride (UF6) into the interior of a vertical, very fast (> 90,000 min-1) rotating cylinder. Under the influence of the high speed and the resulting mass-dependent centrifugal forces, the heavier ones concentrate 238UF6-Molecules on the outer wall of the cylindrical rotor and the lighter ones 235UF6-Molecules close to the rotor axis, which separates the isotopes.
Uranium hexafluoride is also so well suited for the enrichment process because fluorine only occurs in one isotope. The bulk of the UF6-Molecules therefore only varies due to the different masses of the uranium isotopes. Due to the relatively small mass of the fluorine atom, the relative mass difference between the molecules is about 0.85% compared to about 1.3% relative mass difference between the uranium isotopes:
The separation of isotopes is enhanced by creating an axial circulating flow by heating the lower part and cooling the upper part of the centrifuge. The greatest difference in concentration is then no longer between the axis and the rotor wall, but between the ends of the centrifuge. The enriched fraction (“Product”) is removed from the lower end, the depleted fraction (“Tails”) from the upper end of the centrifuge.
Such a centrifuge is also referred to as a countercurrent centrifuge. The extraction tubes for the enriched and depleted fraction protrude into the area of the rotating gas on the outer wall of the centrifuge and thus use the dynamic pressure to transport the gas within the system. In the centrifuge process, too, the separation process takes place under negative pressure, so “Product” and “Tails” must be increased to normal pressure with the help of compressors and sublimers / desublimators before they can be filled into the transport or storage container.
The gas centrifuges are usually connected in cascades with several hundred individual centrifuges, since each centrifuge can only achieve a limited throughput and a limited enrichment. Parallel connection of the centrifuges ensures an increase in throughput, while the enrichment is increased by connecting in series.
The effectiveness of the centrifuges can be increased by increasing the tube length and, in particular, the speed of rotation; they therefore have an elongated, roller-like shape. With aluminum alloys, 400 m / s are achieved, with high-strength steels 500 m / s and with fiber-reinforced materials over 700 m / s. The separation performance is practically limited by the material properties of the rapidly rotating rotor as well as by technical restrictions of the rotor length (occurrence of undesired natural vibrations).
As in a mass spectrometer, in electromagnetic isotope separation, uranium atoms are first ionized, then accelerated in an electric field and then separated according to their mass number in a magnetic field. This isotope separation setup was used in World War II to make enriched uranium for the first atomic bombs; the systems used at that time were called calutrones.
Because of the enormous effort involved, this process is no longer of any importance for the production of enriched uranium. However, it is used in research for other isotope separations, since in the ideal case a single obtained atom of an isotope can be detected.
The laser enrichment is based on the isotope shift of the absorption spectra of atoms and molecules. Are the spectroscopic conditions suitable, i. H. if the absorption lines of the isotopes or isotope compounds overlap sufficiently little and if a laser of suitable wavelength and narrow band is available, an isotope-selective excitation is possible. For the separation, use is then made of the fact that the excited species differs significantly from the non-excited species in terms of their physical and chemical properties. Laser processes are characterized by a high level of selectivity.
Basically, two concepts can be distinguished: the photoionization of uranium vapor (atomic process; AVLIS) and the photodissociation of UF6 (molecular method; MLIS). Theoretically, the laser process allows isotope separation in a single step. In practice, the number of stages required depends on the extent to which the ideal conditions can be achieved.
In the atomic process, the atoms of an isotope mixture are selectively ionized. After the ionization of an isotope (235U) it can easily be removed from the non-ionized atoms of the other isotope (238U) are separated by acceleration in an electric field.
In the molecular process, that becomes 235Molecule containing U is first excited by a first laser before a fluorine atom is split off by a second laser. The emerging solid 235UF5 can be easily filtered from the gas.
After initial euphoria about the advantages of these processes compared to conventional, established enrichment processes, people have now become more skeptical about their industrial feasibility. Many research and development programs have already been discontinued, as it turned out that the technical problems (corrosion on the equipment) are so insurmountable that even high-tech countries fail.
Separation nozzle process
The separation nozzle process was also developed in Germany by the end of the 1980s. Here the segregation of the uranium isotopes takes place due to different centrifugal forces in a fast, curved flow. In 1975, Brazil adopted this process as part of the German-Brazilian nuclear energy agreement in order to process its large uranium deposits; however, the planned systems were not implemented. One of the advantages of the separation nozzle method was: to ensure that it was not subject to any confidentiality restrictions. The Republic of South Africa practically used the separating nozzle method before 1990, because due to the embargo against the country, only techniques could be used that could be used without great difficulty: no confidentiality restrictions by the Federal Republic of Germany. The high energy consumption played no role.
Importance of uranium enrichment for the construction of nuclear weapons
Enrichment of 235U is not a requirement for building nuclear weapons. The graphite or heavy water (D.2O) -moderated reactor by neutron capture of 238The resulting plutonium is also suitable for weapons, but it only enables the construction of comparatively weak nuclear charges, is radioactive, poisonous and difficult to handle. The 239Pu only has to be separated from further decay products by reprocessing: the plutonium obtained in such a reactor can be separated from the other fissile substances in a purely chemical way, see also plutonium bomb. Nuclear reactors, which are mainly weapons-grade 239Generate Pu, have a very simple design, are therefore less suitable for generating electricity, are unsafe and, moreover, can be clearly recognized by anyone skilled in the art as being built for this purpose.
The construction and operation of an enrichment plant requires a much higher technological level than the construction of a simple nuclear reactor for breeding plutonium.
It is also possible to build a large enrichment facility with thousands of centrifuges for the production of highly enriched nuclear weapons 235There's no better way to hide U than building a nuclear reactor.
The military motive of uranium enrichment is that one is highly enriched with 235U can build stronger nuclear charges than with plutonium, which, moreover, are easier to handle and store.
In August 2005, the world public looked at Iran and the controversial restart of its nuclear complex in Natans, Isfahan province. There uranium enrichment is carried out on a comparatively small scale, the degree of enrichment achieved is far from being bomb-proof. However, Iran claims its right to enrichment for civil purposes: a nuclear reactor requires a much lower degree of enrichment than a bomb. However, mastering the gas centrifuge technology for enrichment represents an essential threshold on the way to nuclear power, since the scaling up to larger quantities and degrees of enrichment is only a question of technical resources.
Category: Nuclear Technology
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